Add gcc 15 and clang 21 to linux CI and update actions/checkout#2868

Alex-PLACET wants to merge 1 commit intoxtensor-stack:masterxtensor-stack/xtensor:masterfrom

Alex-PLACET:update_ci_and_add_gcc15_clang21Alex-PLACET/xtensor:update_ci_and_add_gcc15_clang21Copy head branch name to clipboard