ICU-23243 Bugfix: Don't call std::string_view::front() when empty.

Doing this just happens to work in most builds (which is why this bug
hasn't been found until now), but it's actually undefined behavior.

ICU-23056 some ICU 78 download page updates

See #3711

ICU-23121 Export compressibleBytes as bools instead of packed u8

ICU-23054 Bump the github-actions group with 2 updates

See #3490

ICU-22923 Workaround for addLikelySubtags(und-Latn-RS) bug

ICU-22922 ICU 77rc download page

See #3407

ICU-22920 Fix raw type warnings in icu4j core tests

ICU-22724 BRS_76_GA Update version number to 76.1 (No more -SNAPSHOT)

ICU-22723 download 76rc

ICU-22718 Export disallowed/ignored UTS 46 data for ICU4X