Replace `GUARDED_BY` with `ABSL_GUARDED_BY` in preparation for abseil upgrade #2861

Job	Run time
env-protect-setup	11s
authorize	4s
get-dev-image / get-image	5s
clang-tidy	19m 34s
generate-matrix	2m 56s
code-coverage	0s
build-and-test (Clang debug, nokvm, --config=clang --compilation_mode=dbg, bazel_buildables_clang...	6m 23s
build-and-test (BPF opt (4.14.254), kvm, --config=qemu-bpf --compilation_mode=opt --//bazel/test_...	5m 27s
build-and-test (Clang opt + UI, nokvm, --config=clang --compilation_mode=opt, bazel_buildables_cl...	10m 37s
build-and-test (TSAN, nokvm, --config=tsan --compilation_mode=dbg, bazel_buildables_sanitizer, ba...	6m 31s
build-and-test (GCC opt, nokvm, --config=gcc --compilation_mode=opt, bazel_buildables_gcc_opt, ba...	10m 14s
build-and-test (BPF opt (6.1.106), kvm, --config=qemu-bpf --compilation_mode=opt --//bazel/test_r...	5m 11s
build-and-test (Go race detector, nokvm, --config=go_race --compilation_mode=opt, bazel_buildable...	4m 14s
build-and-test (ASAN, nokvm, --config=asan --compilation_mode=dbg, bazel_buildables_sanitizer, ba...	8m 26s
build-and-test-status	2s
	1h 19m 55s

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