Improve migration docs from ORM 2 to 3. (#12207)

* Improve migration docs from ORM 2 to 3.

* Improve wording

* Hint about following deprecations.

Author: beberlei

UPGRADE.md

@@ -1,5 +1,36 @@
+# Upgrade to 3.x General Notes
+
+We recommend you upgrade to DBAL 3 first before upgrading to ORM 3. See
+the DBAL upgrade docs: https://github.com/doctrine/dbal/blob/3.10.x/UPGRADE.md
+
+Rather than doing several major upgrades at once, we recommend you do the following:
+
+- upgrade to DBAL 3
+- deploy and monitor
+- upgrade to ORM 3
+- deploy and monitor
+- upgrade to DBAL 4
+- deploy and monitor
+
+If you are using Symfony, the recommended minimal Doctrine Bundle version is 2.15
+to run with ORM 3.
+
+At this point, we recommend upgrading to PHP 8.4 first and then directly from
+ORM 2.19 to 3.5 and up so that you can skip the lazy ghost proxy generation
+and directly start using native lazy objects.
+
+Note about upgrading: Doctrine uses static and runtime mechanisms to raise
+awareness about deprecated code.
+
+- Use of `@deprecated` docblock that is detected by IDEs (like PHPStorm) or
+  Static Analysis tools (like Psalm, phpstan)
+- Use of our low-overhead runtime deprecation API, details:
+  https://github.com/doctrine/deprecations/
+
 # Upgrade to 3.5
 
+See the General notes to upgrading to 3.x versions above.
+
 ## Deprecate not using native lazy objects on PHP 8.4+
 
 Having native lazy objects disabled on PHP 8.4+ is deprecated and will not be
@@ -53,6 +84,8 @@ decide to remove it before it is used too widely.
 
 # Upgrade to 3.4
 
+See the General notes to upgrading to 3.x versions above.
+
 ## Discriminator Map class duplicates
 
 Using the same class several times in a discriminator map is deprecated.
@@ -73,6 +106,8 @@ Companion accessor methods are deprecated as well.
 
 # Upgrade to 3.3
 
+See the General notes to upgrading to 3.x versions above.
+
 ## Deprecate `DatabaseDriver`
 
 The class `Doctrine\ORM\Mapping\Driver\DatabaseDriver` is deprecated without replacement.
@@ -89,6 +124,8 @@ method. Details can be found at https://github.com/doctrine/orm/pull/11188.
 
 # Upgrade to 3.2
 
+See the General notes to upgrading to 3.x versions above.
+
 ## Deprecate the `NotSupported` exception
 
 The class `Doctrine\ORM\Exception\NotSupported` is deprecated without replacement.
@@ -115,6 +152,8 @@ using the `\Doctrine\ORM\Mapping\UniqueConstraint` and `\Doctrine\ORM\Mapping\In
 
 # Upgrade to 3.1
 
+See the General notes to upgrading to 3.x versions above.
+
 ## Deprecate `Doctrine\ORM\Mapping\ReflectionEnumProperty`
 
 This class is deprecated and will be removed in 4.0.
@@ -138,6 +177,8 @@ Using array access on instances of the following classes is deprecated:
 
 # Upgrade to 3.0
 
+See the General notes to upgrading to 3.x versions above.
+
 ## BC BREAK: Calling `ClassMetadata::getAssociationMappedByTargetField()` with the owning side of an association now throws an exception
 
 Previously, calling
@@ -163,6 +204,9 @@ so `$targetEntity` is a first argument now. This change affects only non-named a
 When using the `AUTO` strategy to let Doctrine determine the identity generation mechanism for
 an entity, and when using `doctrine/dbal` 4, PostgreSQL now uses `IDENTITY`
 instead of `SEQUENCE` or `SERIAL`.
+
+There are three ways to handle this change.
+
 * If you want to upgrade your existing tables to identity columns, you will need to follow [migration to identity columns on PostgreSQL](https://www.doctrine-project.org/projects/doctrine-dbal/en/4.0/how-to/postgresql-identity-migration.html)
 * If you want to keep using SQL sequences, you need to configure the ORM this way:
 ```php
@@ -175,6 +219,27 @@ $configuration->setIdentityGenerationPreferences([
     PostgreSQLPlatform::CLASS => ClassMetadata::GENERATOR_TYPE_SEQUENCE,
 ]);
 ```
+* You can change individual entities to use the `SEQUENCE` strategy instead of `AUTO`:
+```php
+
+diff --git a/src/Entity/Example.php b/src/Entity/Example.php
+index 28be8df378..3b7d61bda6 100644
+--- a/src/Entity/Example.php
++++ b/src/Entity/Example.php
+@@ -38,7 +38,7 @@ class Example
+
+     #[ORM\Id]
+     #[ORM\Column(type: 'integer')]
+-    #[ORM\GeneratedValue(strategy: 'AUTO')]
++    #[ORM\GeneratedValue(strategy: 'SEQUENCE')]
+     private int $id;
+
+     #[Assert\Length(max: 255)]
+```
+The later two options require a small database migration that will remove the default
+expression fetching the next value from the sequence. It's not strictly necessary to
+do this migration because the code will work anyway. A benefit of this approach is
+that you can just make and roll out the code changes first and then migrate the database later.
 
 ## BC BREAK: Throw exceptions when using illegal attributes on Embeddable