Modify nox session release:prepare to add a summary section with resolved security issues

[ArBridgeman]

Summary

As preparation for the completion of #371, the PTB should prepend a summary section with resolved security issues onto the versioned changes file.

Tasks

• Move & refactor Class Audit to exasol/toolbox/util/dependencies/audit.py
• Create a setup similar to track_changes but for obtaining vulnerability fixes since the latest tag
  • the analog to DependencyChange should have two conversions to strings: 1) for in the Summary section and 2) for adding to the Security section later
  • the analog to DependencyChanges should look to see what vulnerabilities in the previous version were fixed in the current one (so we're only considering one side of the equation)
• (optional) consider splitting off the functions for the versioned changes file to a separate class, as mostly, we're adapting that, but maybe there's another smart refactoring to better show encapsulation or moving out functions too
• Adapt class Changelogs to prepend the summary with security_issues on the versioned changes file
• Test these new features
• Adapt the documentation https://exasol.github.io/python-toolbox/main/user_guide/features/creating_a_release.html#preparing-a-release with a line about including security fixes in the summary

[ArBridgeman]

relevant links:
https://github.com/exasol/databricks-virtual-schema/pull/54/files
https://github.com/exasol/project-keeper/blob/main/doc/changes/changes_4.3.1.md
https://github.com/exasol/project-keeper/blob/main/doc/changes/changes_4.4.0.md
https://github.com/exasol/project-keeper/blob/main/doc/requirements/design.md#pk-mode-verify-release-verifies-changes-file