More "semantic-versioning" compatible check for back-end schema

Summary

This MR refactors the checks for the mysql schema version.

Test Environment

Just put different values for MYSQL_SCHEMA_VERSION.

Check List for the Author

• All automated tests pass
• Reference related Issues
• Annotations in code (Gitlab comments)
  • Intent of new code
  • Problems with old code
  • Why this implementation?

Check List for the Reviewer

• I understand the intent of this MR
• All automated tests pass
• The test environment setup works and the intended behavior is reproducible in the test environment
• In-code documentation and comments are up-to-date.

mentioned in issue caosdb-mysqlbackend#31 (closed)

changed the description

marked the checklist item All automated tests pass as completed

marked the checklist item Reference related Issues as completed

marked the checklist item Annotations in code (Gitlab comments) as completed

requested review from @daniel

added 1 commit

• 7a572155 - DOC: Documentation and small refactoring.

Compare with previous version

resolved all threads

marked the checklist item I understand the intent of this MR as completed

marked the checklist item In-code documentation and comments are up-to-date. as completed

merged

mentioned in commit 34fdd6fb