Refresh .gitattributes + accept *.x branches in Stryker config

.gitattributes
- Drop dead entries (/etc, /docs, /behat.yml.dist,
  /composer-require-checker.json, /psalm-baseline.xml, /psalm.xml,
  /CHANGELOG.md) — none of those files exist in the repo anymore.
- Add the current tooling configs that should stay out of the Composer
  tarball: /CLAUDE.md, /composer-dependency-analyser.php,
  /infection.json5, /phpstan.neon, /rector.php.

infection.json5
- Swap the Stryker badge match from the literal "master" to the regex
  /^\d+\.x$/, so pushes to 1.x and 2.x both publish to the dashboard.
  The literal-string matcher was silently dropping every report with
  the warning "Branch \"2.x\" does not match expected Stryker
  configuration".

Author: loevgaard

.gitattributes

@@ -1,18 +1,15 @@
-/.github                        export-ignore
-/etc                            export-ignore
-/docs                           export-ignore
-/.editorconfig                  export-ignore
-/.gitattributes                 export-ignore
-/.gitignore                     export-ignore
-/README.md                      export-ignore
-/behat.yml.dist                 export-ignore
-/composer-require-checker.json  export-ignore
-/docs                           export-ignore
-/ecs.php                        export-ignore
-/node_modules                   export-ignore
-/phpunit.xml.dist               export-ignore
-/psalm-baseline.xml             export-ignore
-/psalm.xml                      export-ignore
-/tests                          export-ignore
-/CHANGELOG.md                   export-ignore
-/UPGRADE.md                     export-ignore
+/.editorconfig                       export-ignore
+/.github                             export-ignore
+/.gitattributes                      export-ignore
+/.gitignore                          export-ignore
+/CLAUDE.md                           export-ignore
+/README.md                           export-ignore
+/UPGRADE.md                          export-ignore
+/composer-dependency-analyser.php    export-ignore
+/ecs.php                             export-ignore
+/infection.json5                     export-ignore
+/node_modules                        export-ignore
+/phpstan.neon                        export-ignore
+/phpunit.xml.dist                    export-ignore
+/rector.php                          export-ignore
+/tests                               export-ignore

infection.json5

@@ -8,7 +8,10 @@
         "text": "php://stderr",
         "github": true,
         "stryker": {
-            "badge": "master"
+            // Regex (delimited by `/`) so every `<n>.x` maintenance branch — currently 1.x and 2.x —
+            // updates the Stryker dashboard badge. A literal string here is matched exactly, so
+            // pinning to a single branch name silently drops the report for every other one.
+            "badge": "/^\\d+\\.x$/"
         }
     },
     "minMsi": 51.02,