docs: Document howto upgrade from generic python plugin

[bruno-at-bareos]

This PR add instructions to Update Bareos chapter about python plugin broken packages with apt upgrade

[OP #5250] [OP #5155]

Please check

• [X] Short description and the purpose of this PR is present above this paragraph
• [X] Your name is present in the AUTHORS file (optional)

If you have any questions or problems, please give a comment in the PR.

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Checklist for the reviewer of the PR (will be processed by the Bareos team)

General

• [ ] PR name is meaningful
• [ ] Purpose of the PR is understood
• [ ] Commit descriptions are understandable and well formatted
• [ ] Check backport line
• [ ] Is the PR title usable as CHANGELOG entry?
• [ ] Separate commit for CHANGELOG.md ("update CHANGELOG.md"). The PR number is correct.

Source code quality

• [ ] Source code changes are understandable
• [ ] Variable and function names are meaningful
• [ ] Code comments are correct (logically and spelling)
• [ ] Required documentation changes are present and part of the PR
• [ ] bareos-check-sources --since-merge does not report any problems

Tests

• [ ] Decision taken that a test is required (if not, then remove this paragraph)
• [ ] The choice of the type of test (unit test or systemtest) is reasonable
• [ ] Testname matches exactly what is being tested
• [ ] On a fail, output of the test leads quickly to the origin of the fault