gh-145098: Use macos-15-intel instead of unstable macos-26-intel

[hugovk]

The macos-26-intel appears to be unstable, with unexplained hangs:

• CI: move Apple Silicon macOS workflows to macos-26 (Tahoe) #145098 (comment)
• CI: move Apple Silicon macOS workflows to macos-26 (Tahoe) #145098 (comment)

This seems unrelated to the Tk hangs (#146531) because it also happens when skipping the Tk test on macos-26-intel:

• gh-146531: Skip hanging wm iconbitmap test on macOS 26 Intel #148032
• https://github.com/python/cpython/actions/runs/23944426082/job/69837525233?pr=148032

This PR changes the main tests from macos-26-intel to macos-15-intel.

It doesn't change it in jit.yml or tail-call.yml as they seem okay.

• Issue: CI: move Apple Silicon macOS workflows to macos-26 (Tahoe) #145098

[webknjaz]

FTR, we've seen some GHA instability in pip-tools, with macos 26 specifically, just the other day. So that might be a platform issue.

cc @sirosen

[hugovk]

It appears so, see actions/runner-images#13882 for another report.