Add additional validation to secret_suffix in Kubernetes backend

[bschaatsbergen]

Addresses an issue with the secret_suffix attribute in the Kubernetes backend. It adds a check to prevent suffixes from ending with a -<number>, as this conflicts with the backend’s chunking mechanism that appends numeric indices to secret names.

Details on the exact underlying issue and chunking mechanism can be found here : #35641 (comment)

Fixes #35641

[alexsomesan]

This looks good to me, following our conversation in Slack.

Deferring to @jrhouston for a second pair of eyes and final approval as he's more familiar with this code than I am.

[jrhouston]

Thanks for adding validation for this @bschaatsbergen – I'm going to approve, but I've left you some comments proposing some cosmetic changes to the code.

[github-actions]

Reminder for the merging maintainer: if this is a user-visible change, please update the changelog on the appropriate release branch.

[github-actions]

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