[icu] Update icu to 12de966fcbe1d1a48dba310aee63807856ffeee8

[filmil]

Issue: flutter/flutter#105342

Rolls a very new version of the ICU library.

Fixes: flutter/flutter#87073

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide and the C++, Objective-C, Java style guides.
• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test-exempt. See testing the engine for instructions on
  writing and running engine tests.
• I updated/added relevant documentation (doc comments with ///).
• I signed the CLA.
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

[filmil]

Needs an update of the license parsing code. I think we had this somewhere in the prior attempt at updating this, so I will try to go there and see how the fix was done.

[filmil]

#33274 has the needed license fixes, I think.

[filmil]

Correction: #32879

[filmil]

The change that is being checked now has a chance of passing all the licensing checks.

[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 Hixie on the #hackers channel in Chat (don't just cc him here, he won't see it! He's on 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.

[filmil]

@jason-simmons Would you mind approving this for merge if you are OK with the change?

[fluttergithubbot]

This pull request is not suitable for automatic merging in its current state.

• Please get at least one approved review if you are already a member or two member reviews if you are not a member before re-applying this label. Reviewers: If you left a comment approving, please use the "approve" review action instead.