graphics: drop MIR_EXPERIMENTAL_HYBRID_GRAPHICS and use the virtual platform in CI

[Saviq]

No description provided.

[Saviq]

Needed to fix:

• "Client has not exited: Terminating client" in smoke tests on virtual display #3102
• The compositor report doesn't work on the virtual platform #3101

[AlanGriffiths]

* [The compositor report doesn't work on the virtual platform #3101](https://github.com/MirServer/mir/issues/3101)

I think we can drop this test, it isn't relevant anymore

[Saviq]

I think we can drop this test, it isn't relevant anymore

Well, it's as relevant as it was before? Just not on the virtual platform? :)

Would still be good to run it to include its coverage, but yeah that's not a priority.

[AlanGriffiths]

Well, it's as relevant as it was before? Just not on the virtual platform? :)

It checks that we don't exhaust the frontend thread pool by starting "many" clients. That hasn't been since we dropped mirclient (and not actually possible for a long time before that).

[codecov]

Codecov Report

Merging #3099 (0e1a786) into bugfix/3102 (389033c) will decrease coverage by 0.53%.
Report is 1 commits behind head on bugfix/3102.
The diff coverage is n/a.

@@               Coverage Diff               @@
##           bugfix/3102    #3099      +/-   ##
===============================================
- Coverage        78.32%   77.80%   -0.53%     
===============================================
  Files             1074     1064      -10     
  Lines            74793    74359     -434     
===============================================
- Hits             58583    57853     -730     
- Misses           16210    16506     +296     

Files	Coverage Δ
src/server/graphics/default_configuration.cpp	79.73% <ø> (-1.02%)	⬇️

... and 50 files with indirect coverage changes

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[AlanGriffiths]

No known blockers now