Name the release line's branch for the GitHub release

JReleaser takes main as release.github.branch unless told otherwise,
and before it opens a release it looks the branch up on GitHub. A
branch that is not there it creates with git push --all, which from
the tag's detached checkout has nothing to push. With main gone from
GitHub, the draft release of 21.0.0 failed that way, "Could not
checkout branch main", while Maven Central, the Gradle Plugin Portal
and the JetBrains Marketplace got the version.

The branch is now 21.X.X, the current release line; it moves when the
next line starts, as target-branch does in dependabot.yml. A dry run
skips the lookup, so only the next tag proves the fix; jreleaserConfig
shows branch and branchPush as 21.X.X.

Author: abashev

buildSrc/src/main/groovy/open-java-format.release-conventions.gradle

@@ -88,6 +88,9 @@ jreleaser {
             skipTag = true
             tagName = '{{projectVersion}}'
             releaseName = '{{projectVersion}}'
+            // The current release line; it moves when the next line starts. JReleaser assumes main otherwise,
+            // and when GitHub has no such branch it tries to push one, which fails from the tag's checkout.
+            branch = '21.X.X'
             // Published by hand after a look.
             draft = true
             changelog {