Cap boto3 in release-python-runtime job #11254

Job	Run time
fixup / block-fixup	7s
lint / bazel	2m 1s
labels / InternalPRRequired	1s
test (linux, ubuntu-22.04) / bazel	2m 10s
test (linux, ubuntu-22.04, -debug) / bazel	3m 20s
test (linux, ubuntu-22.04, -asan) / bazel	2m 12s
test (macOS, macos-15) / bazel	2m 21s
test (windows, windows-2025) / bazel	13m 50s
	26m 2s

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