Tooling: allow alpha release suffix on release branches

[tbradsha]

Currently when creating a release branch, it removes any suffix starting with a hyphen. This was probably intended to remove the -beta suffix (p1725907150549519/1725905587.302229-slack-C05Q5HSS013) but now that Jetpack no longer creates weekly/* branches, it uses -a.* as the base branch, and the code strips said suffix. This is means each alpha release tries to push to the same branch rather than an alpha-specific branch.

This PR is quite simple: it switches the code to only remove the -beta suffix.

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Jetpack product discussion

Does this pull request change what data or activity we track or use?

Testing instructions:

Verify the following create branches matching the passed version (no need to checkout prerelease):

• tools/create-release-branch.sh jetpack 14.1
• tools/create-release-branch.sh jetpack 14.1-a.5
• tools/create-release-branch.sh jetpack 14.1-beta

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[anomiex]

At some point we may want to revisit this to decide if we want to also remove -beta.2 and the like.