Change selfhosted version numbering

Summary

Change selfhosted version numbering to make it NextCloud-independant.

Description

Selfhosted is not only NextCloud, so it's version numbering shouldn't rely on it.

Who will use this new feature?

/e/ Team and contributors may want to update components beside NextCloud.
In current scheme, it's not easy as selfhosted version relies only on NextCloud.

Why these users would like to use this feature?

Anyone may need to upgrade another component (i.e. welcome) or apply a fix.

Examples

I'm currently working on implementing MTA-STS. What would be the selfhosted version tag, as there is no NextCloud update ?

Reflection

Best would be a "classic" version number, i.e. V1R2. One may want to start at V24 to hang up with current numbering.

Validation

No testing needed.