GHA: Add GCC 15.2 & 16, and Clang 21 & 22, and GCC 16 w/ reflection

[Flamefire]

Test job https://github.com/boostorg/boost-ci/actions/runs/24935283953

Replaces the GCC 15.0 compiler from ubuntu 25.04 (non-LTS) by GCC 25.2 from 26.04 LTS

Also adds GCC 16 with and without reflection enabled and enhances the Boost.CI test to verify it works. E.g. the clang-based reflection job (likely) required the reflection flag.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (36cb137) to head (081b6b2).
⚠️ Report is 9 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #327   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            3         3           
  Lines           23        24    +1     
  Branches        10        11    +1     
=========================================
+ Hits            23        24    +1     

Files with missing lines	Coverage Δ
test/test.cpp	100.00% <100.00%> (ø)

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[sdarwin]

If you have tested this, and it works, then sure. Why not.