ci.yml: fix push branch filter pattern (backport #502)

[mergify]

🦟 Bug fix

Fixes triggering of GitHub workflows on branches with double-digit major version.

Summary

This fixes the branch regex to run Ubuntu workflows on push to branches with a double digit major version number. Similar to gazebosim/sdformat#1445.

Checklist

• Signed all commits for DCO
• Added a screen capture or video to the PR description that demonstrates the fix (as needed)
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• Updated Bazel files (if adding new files). Created an issue otherwise.
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This is an automatic backport of pull request #502 done by [Mergify](https://mergify.com).