Replace `GUARDED_BY` with `ABSL_GUARDED_BY` in preparation for abseil upgrade #2865

Job	Run time
env-protect-setup	7s
authorize	2s
get-dev-image / get-image	7s
clang-tidy	24m 58s
generate-matrix	3m 5s
code-coverage	0s
build-and-test (TSAN, nokvm, --config=tsan --compilation_mode=dbg, bazel_buildables_sanitizer, ba...	14m 10s
build-and-test (Clang opt + UI, nokvm, --config=clang --compilation_mode=opt, bazel_buildables_cl...	9m 5s
build-and-test (Go race detector, nokvm, --config=go_race --compilation_mode=opt, bazel_buildable...	4m 12s
build-and-test (GCC opt, nokvm, --config=gcc --compilation_mode=opt, bazel_buildables_gcc_opt, ba...	9m 52s
build-and-test (ASAN, nokvm, --config=asan --compilation_mode=dbg, bazel_buildables_sanitizer, ba...	19m 11s
build-and-test (Clang debug, nokvm, --config=clang --compilation_mode=dbg, bazel_buildables_clang...	14m 5s
build-and-test-status	4s
	1h 38m 58s

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