Update CI to run on macos-15

[Luthaf]

Description

macos-13 runners are no longer available, and should be replaced.

@Iximiel in #1347 you changed macos-13 to macos-15, but also kept a macos-14 runner. Both will run on ARM CPU, is this intentional or should I change one to run on Intel CPU?

Target release

I would like my code to appear in release v2.10

Type of contribution

• changes to code or doc authored by PLUMED developers, or additions of code in the core or within the default modules
• changes to a module not authored by you
• new module contribution or edit of a module authored by you

Copyright

• I agree to transfer the copyright of the code I have written to the PLUMED developers or to the author of the code I am modifying.

• the module I added or modified contains a COPYRIGHT file with the correct license information. Code should be released under an open source license. I also used the command cd src && ./header.sh mymodulename in order to make sure the headers of the module are correct.

Tests

• I added a new regtest or modified an existing regtest to validate my changes.
• I verified that all regtests are passed successfully on GitHub Actions.

[Iximiel]

It looked like conda having a beef with the intel mac15, I need to find the CI log, it was something like trying to use python2.7 in the requisite.

Since it was the same problem that appeared with mac13, and I do not have local machine to test, I inferred that the culprit in failing the CI was the "intel" part of the runners. So I gave up in finding a solution

[Luthaf]

Fair enough, should I remove the macos-14 runner and only keep macos-15 then?

[Iximiel]

Fair enough, should I remove the macos-14 runner and only keep macos-15 then?

this is a question for @GiovanniBussi