Bump itertools from 0.14.0 to 0.15.0

[cakebaker]

This PR bumps itertools from 0.14.0 to 0.15.0 and adds it to the skip list in deny.toml.

Closes #12928

[github-actions]

GNU testsuite comparison:

Skipping an intermittent issue tests/cut/bounded-memory (passes in this run but fails in the 'main' branch)
Skipping an intermittent issue tests/pr/bounded-memory (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/tail/tail-n0f is now being skipped but was previously passing.

[oech3]

Is this crate still useful?

[cakebaker]

I don't know, it's probably more of a "nice to have" than a "must have" crate.