fix(ci): grant contents:write permission to version-bump workflow

Caller workflow had no permissions block, so GitHub's permission-
intersection rule capped the nested reusable-version-bump.yml call
to read-only, breaking dispatch ("requesting 'contents: write', but
is only allowed 'contents: read'").

[skip ci]

.github/workflows/version-bump.yml

@@ -1,5 +1,8 @@
 name: Version Bump
 
+permissions:
+  contents: write
+
 on:
   pull_request:
     types: [closed]