Adjusted the BC rules to be consistent

A previous rules states that we guarantee BC for code extending the class. This rules does not allow marking a class as final as it would be a fatal error for extending code after the upgrade, which is far from being BC.
minor #3645 Adjusted the BC rules to be consistent (stof)

This PR was merged into the 2.3 branch.

Discussion
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Adjusted the BC rules to be consistent

A previous rule states that we guarantee BC for code extending the class. This rules does not allow marking a class as final as it would be a fatal error for extending code after the upgrade, which is far from being BC.

/cc @fabpot @webmozart

Commits
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6c1ded9 Adjusted the BC rules to be consistent