minor #23031 [Yaml] Clarify "incompatible key casting" deprecation message (rvanlaak)

This PR was squashed before being merged into the 3.3 branch (closes #23031).

Discussion
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[Yaml] Clarify "incompatible key casting" deprecation message

| Q             | A
| ------------- | ---
| Branch?       | 3.3
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | N/A
| License       | MIT
| Doc PR        | N/A

In the process of upgrading to Symfony 3.3 our Yaml linter tests started throwing errors. The exception was a bit unclear to us, so hope this message will help others with fixing their deprecations as well.

Commits
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67895f4dd3 [Yaml] Clarify "incompatible key casting" deprecation message

Author: fabpot

Parser.php

@@ -237,7 +237,8 @@ private function doParse($value, $flags)
                 }
 
                 if (!(Yaml::PARSE_KEYS_AS_STRINGS & $flags) && !is_string($key)) {
-                    @trigger_error('Implicit casting of incompatible mapping keys to strings is deprecated since version 3.3 and will throw \Symfony\Component\Yaml\Exception\ParseException in 4.0. Pass the PARSE_KEYS_AS_STRING flag to explicitly enable the type casts.', E_USER_DEPRECATED);
+                    $keyType = is_numeric($key) ? 'numeric key' : 'incompatible key type';
+                    @trigger_error(sprintf('Implicit casting of %s to string on line %d is deprecated since version 3.3 and will throw \Symfony\Component\Yaml\Exception\ParseException in 4.0. Pass the PARSE_KEYS_AS_STRING flag to explicitly enable the type casts.', $keyType, $this->getRealCurrentLineNb()), E_USER_DEPRECATED);
                 }
 
                 // Convert float keys to strings, to avoid being converted to integers by PHP

Tests/ParserTest.php

@@ -1081,7 +1081,7 @@ public function testYamlDirective()
 
     /**
      * @group legacy
-     * @expectedDeprecation Implicit casting of incompatible mapping keys to strings is deprecated since version 3.3 and will throw \Symfony\Component\Yaml\Exception\ParseException in 4.0. Pass the PARSE_KEYS_AS_STRING flag to explicitly enable the type casts.
+     * @expectedDeprecation Implicit casting of numeric key to string on line 1 is deprecated since version 3.3 and will throw \Symfony\Component\Yaml\Exception\ParseException in 4.0. Pass the PARSE_KEYS_AS_STRING flag to explicitly enable the type casts.
      */
     public function testFloatKeys()
     {
@@ -1103,7 +1103,7 @@ public function testFloatKeys()
 
     /**
      * @group legacy
-     * @expectedDeprecation Implicit casting of incompatible mapping keys to strings is deprecated since version 3.3 and will throw \Symfony\Component\Yaml\Exception\ParseException in 4.0. Pass the PARSE_KEYS_AS_STRING flag to explicitly enable the type casts.
+     * @expectedDeprecation Implicit casting of incompatible key type to string on line 1 is deprecated since version 3.3 and will throw \Symfony\Component\Yaml\Exception\ParseException in 4.0. Pass the PARSE_KEYS_AS_STRING flag to explicitly enable the type casts.
      */
     public function testBooleanKeys()
     {