TASK: Add release notes for 9.1 [skip ci]

See https://jenkins.neos.io/job/flow-release/483/

Author: Jenkins

Neos.Flow/Documentation/TheDefinitiveGuide/PartV/ReleaseNotes/910.rst

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+========
+Flow 9.1
+========
+
+This release of Flow comes with some great new features, bugfixes and a lot of modernisation of the existing code base.
+
+As usual, we worked hard to keep this release as backwards compatible as possible but, since it's a major release, some of the changes might require manual
+adjustments. So please make sure to carefully read the upgrade instructions below.
+
+
+************
+New Features
+************
+
+
+********************
+Upgrade Instructions
+********************
+
+This section contains instructions for upgrading your Flow **9.0**
+based applications to Flow **9.1**.
+
+In general just make sure to run the following commands:
+
+To clear all file caches::
+
+ ./flow flow:cache:flush --force
+
+If you have additional cache backends configured, make sure to flush them too.
+
+To apply core migrations::
+
+  ./flow flow:core:migrate <Package-Key>
+
+For every package you have control over (see `Upgrading existing code`_ below).
+
+To validate/fix the database encoding, apply pending migrations and to (re)publish file resources::
+
+ ./flow database:setcharset
+ ./flow doctrine:migrate
+ ./flow resource:publish
+
+If you are upgrading from a lower version than 9.0, be sure to read the
+upgrade instructions from the previous Release Notes first.
+
+Upgrading your Packages
+=======================
+
+Upgrading existing code
+-----------------------
+
+There have been major API changes in Flow 9.1 which require your code to be adjusted. As with earlier changes to Flow
+that required code changes on the user side we provide a code migration tool.
+
+Given you have a Flow system with your (outdated) package in place you should run the following before attempting to fix
+anything by hand::
+
+ ./flow core:migrate Acme.Demo
+
+This will adjust the package code automatically and/or output further information.
+Read the output carefully and manually adjust the code if needed.
+
+To see all the other helpful options this command provides, make sure to run::
+
+ ./flow help core:migrate
+
+Also make sure to read about the `Potentially breaking changes`_ below.
+
+Inside core:migrate
+^^^^^^^^^^^^^^^^^^^
+
+The tool roughly works like this:
+
+* Collect all code migrations from packages
+
+* Collect all files from the specified package
+* For each migration
+
+  * Check for clean git working copy (otherwise skip it)
+  * Check if migration is needed (looks for Migration footers in commit messages)
+  * Apply migration and commit the changes
+
+Afterwards you probably get a list of warnings and notes from the
+migrations, check those to see if anything needs to be done manually.
+
+Check the created commits and feel free to amend as needed, should
+things be missing or wrong. The only thing you must keep in place from
+the generated commits is the migration data in ``composer.json``. It is
+used to detect if a migration has been applied already, so if you drop
+it, things might get out of hands in the future.