Better previous-pr behavior

[pwtyler]

Checking for any branch that matches prefix means that if this action ran in the past and a PR was closed, the next time WordPress is upgraded, it happily does nothing. Past PRs are closed in favor of the new one. PRs for the same version are a no-op success.

#18 was created from an old version of the project, trying again. Addresses #16.

[jazzsequence]

I think the idea was that the old branch should be deleted after it's merged. In case of a clash, if the versions do not match, I would prefer if the older branch is deleted in favor of the newer branch. e.g.

Current WordPress Version: 6.8.1
Found 'Tested up to' version in readme.txt: 6.7.2
Tested up to version (6.7.2) is less than current WordPress version (6.8.1).
Updating files with new Tested up to version.
Checking if a branch with the name update-tested-up-to-version-6.7.2 exists.
A branch with the name update-tested-up-to-version-6.7.2 exists. Deleting outdated branch and creating new branch.
Branch update-tested-up-to-version-6.8.1 created.
Updating 'Tested up to' version in /home/runner/work/wp-redis/wp-redis/./readme.txt
Updating 'Tested up to' version in /home/runner/work/wp-redis/wp-redis/./README.md
...

If we've gotten to the step where the versions are compared, if the versions don't match, we should delete the older one. Maybe there should be a check to ensure that the version is actually < the newer version. I don't like the idea of the action creating a whole bunch of new branches that never get cleaned out.