[ci] Work around Google Maps podspec lint issue

[stuartmorgan]

#5873 removed a plugin-level workaround for a podspec-lint issue, under the incorrect belief that it was no longer necessary. Rather than re-add that, which degrades the experience for plugin clients who are targeting iOS 13+ with their apps, this adds a hack in the CI tooling itself to modify the podspec to require iOS 13 before linting.

This could in theory cause false-positive lints about deprecated APIs, but currently doesn't, and that could be addressed in the unlikely event that it comes up.

See flutter/flutter#94491

[flutter-dashboard]

It looks like this pull request may not have tests. Please make sure to add tests before merging. If you need an exemption to this rule, contact "@test-exemption-reviewer" in the #hackers channel in Chat (don't just cc them here, they won't see it! Use Discord!).

If you are not sure if you need tests, consider this rule of thumb: the purpose of a test is to make sure someone doesn't accidentally revert the fix. Ask yourself, is there anything in your PR that you feel it is important we not accidentally revert back to how it was before your fix?

Reviewers: Read the Tree Hygiene page and make sure this patch meets those guidelines before LGTMing.

[stuartmorgan]

test-exempt: is a test

(I did consider writing unit tests for this, but since the only purpose of this code is to make flutter/flutter#141493 work, I don't see any value to doing so: either it works and the CI we need is green, or it doesn't and the CI we need will be red.)

[stuartmorgan]

Never mind; we have to put the plugin workaround back. I'll update the issue.