[4472][iOS][infra] Run iOS tests on Mac 11 machines

Change from current Mac 10.15 or Mac 11 config to Mac 11 only.

To sheriffs: This CL is correcting the builder's config. If additional
test failures are observed from the CL, please consider disabling the
test/suite instead of reverting this CL.

No-try: True
Bug: 1215555
Change-Id: I8959c8e0341410fecf1d0fed126fa9b9e19f5df5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2945658
Commit-Queue: Zhaoyang Li <zhaoyangli@chromium.org>
Reviewed-by: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/branch-heads/4472@{#1458}
Cr-Branched-From: 3d60439-refs/heads/master@{#870763}