chore: bump version to 13.7.1 (#723)

## Summary

Version bump to 13.7.1 because v13.7.0's GitHub release became immutable
after a manual `gh release create` conflicted with the automated Release
workflow.

**Lesson learned:** Never create releases manually — the Release
workflow handles everything (archive, SBOM, cosign, release notes) when
triggered by a tag push. Just `git tag -s` + `git push`.

After merge: tag v13.7.1 and let the workflow do its thing.

Author: CybotTM

ext_emconf.php

@@ -21,7 +21,7 @@
     'author_email'   => 'sebastian.koschel@netresearch.de, sebastian.mendel@netresearch.de, rico.sonntag@netresearch.de',
     'author_company' => 'Netresearch DTT GmbH',
     'state'          => 'stable',
-    'version'        => '13.7.0',
+    'version'        => '13.7.1',
     'constraints'    => [
         'depends' => [
             'php'          => '8.2.0-8.9.99',