Update linux-ci to move from macos-13 to macos-15-intel

[jhmgoossens]

See also COIN-OR-OptimizationSuite Issue 35:

The macOS-13 runner image for Actions has been deprecated since 4 DEC 2025, failing runs since then.
This PR replaces the two macos-13 runners (for clang and gcc-13) in linux-ci with two macos-15-intel runners (for clang and gcc-15).

See also more information on GitHub-hosted runners.

[jhmgoossens]

@tkralphs I merely changed macos-13 to macos-15-intel and gcc-13 to gcc-15.
There may be other changes to be done to the CI workflows, but I'd like to get these macos-13 changes implemented so that PR runs work again. Any comments?

[tkralphs]

Looks fine to me except for the issue you brought up in #256. Should we just wait for a resolution there?

[jhmgoossens]

Looks fine to me except for the issue you brought up in #256. Should we just wait for a resolution there?

Thanks. For here (CoinUtils), I propose not to wait and commit the macos-13 fix, but wait with all the other PRs (for Cbc etc.) until #256 is addressed.

If I dont fix the macos-13, then at some point the PR that would fix #256 would not have all Checks Pass (the linux checks would fail)--which is not how I want to advertise the use of PRs...

[jhmgoossens]

The remaining failed Checks are not due to this PR but to #256