Update linux-ci to trim the number of macos builds

[jhmgoossens]

Reduce the number of MaxOS builds to only the two most recent MacOS, and intel only for the latest MacOS:

       - os: macos-26-intel
          flags: CC=clang CXX=clang++
       - os: macos-26
          flags: CC=gcc-15 CXX=g++-15 ADD_CXXFLAGS=-Wl,-ld_classic
       - os: macos-15
          flags: CC=clang CXX=clang++

See also COIN-OR-OptimizationSuite issue 36 discussion.
At the time of writing, the "macos-latest" workflow label resolves to macos-15.

[jhmgoossens]

Unfortunately, all MSVC-related builds/checks fail because of recent CoinTime changes, not due to the CI changes in this PR.