Remove v23.1 from the PR template.

[piyushredpanda]

Remove v23.1.x backport option as v23.1.x goes out of support.

Backports Required

• none - not a bug fix
• none - this is a backport
• none - issue does not exist in previous branches
• none - papercut/not impactful enough to backport
• v23.3.x
• v23.2.x
• v23.1.x

Release Notes

• none

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