Drop old boostrap verions themes, only keep bootstrap 5

Is your feature request related to a problem? Please describe. Bootstrap in its versions 3 and 4 reached end of life (v3 13years ago and v4 8 years ago). We should probably drop support for it : do not provide these themes anymore, nor accepting PR for it.

Describe the solution you'd like Remove the bootstrap 3 and 4 themes. In a major release ? Or a minor is good enough (8 years for the v4 one !) ?

Describe alternatives you've considered No real alternative solution

Additional context .

It could be as good first issue, but since it is so easy, I can handle it myself.