ICU: Fix "GMT+0" in "en-GB" locale

Timezone representation is "GMT+0" in "en-GB" locale since ICU 78
though it was "GMT". It caused a site compatibility issue.

This CL imports Firefox's local patch.
See https://bugzilla.mozilla.org/show_bug.cgi?id=2018880 .

Bug: 473861785
Change-Id: I1ee76235d4ee5a864fc94a980a88dff4941748b3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/deps/icu/+/7719979
Reviewed-by: Dominik Röttsches <drott@chromium.org>