Cleanup dependabot.yaml by 3.13.x configurations

[Zerpet]

Proposed Changes

Removed GitHub Actions and Maven dependency configurations for v3.13.x.

Because 3.13 is out of support.

Types of Changes

What types of changes does your code introduce to this project?
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• Bug fix (non-breaking change which fixes issue #NNNN)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause an observable behavior change in existing systems)
• Documentation improvements (corrections, new content, etc)
• Cosmetic change (whitespace, formatting, etc)
• Build system and/or CI

Checklist

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You can also fill these out after creating the PR.
This is simply a reminder of what we are going to look for before merging your code.

• Mandatory: I (or my employer/client) have have signed the CA (see https://github.com/rabbitmq/cla)
• I have read the CONTRIBUTING.md document
• I have added tests that prove my fix is effective or that my feature works
• All tests pass locally with my changes
• If relevant, I have added necessary documentation to https://github.com/rabbitmq/rabbitmq-website
• If relevant, I have added this change to the first version(s) in release-notes that I expect to introduce it

Further Comments

If this is a relatively large or complex change, kick off the discussion by explaining why you chose the solution
you did and what alternatives you considered, etc.