Towards `setup-gap@v3`

[fingolfin]

We should clean up and simplify a bunch of things

• GAP_PKGS_TO_BUILD and GAP_PKGS_TO_CLONE should go
  • users can achieve this with dedicated build steps
  • we could also add a dedicated action for installing GAP packages (and building if necessary, including dependencies?)
• remove HPCGAP (it can also be achieved by CONFIGFLAGS)
• resolve #26
• resolve #24 by removing (?) the GAPBRANCH input in favor of a new version input (we can still allow passing branches as fallbacks)