chore: use 21.7

Author: blakeli0

.cloudbuild/library_generation/library_generation.Dockerfile

@@ -37,7 +37,7 @@ RUN cp "/root/.m2/repository/com/google/api/gapic-generator-java/${DOCKER_GAPIC_
 FROM docker.io/library/python:3.13.2-slim@sha256:6b3223eb4d93718828223966ad316909c39813dee3ee9395204940500792b740 as final
 
 ARG OWLBOT_CLI_COMMITTISH=3a68a9c0de318784b3aefadcc502a6521b3f1bc5
-ARG PROTOC_VERSION=3.0.0
+ARG PROTOC_VERSION=21.7
 ARG GRPC_VERSION=1.76.1
 ENV HOME=/home
 ENV OS_ARCHITECTURE="linux-x86_64"

.cloudbuild/library_generation/library_generation_airlock.Dockerfile

@@ -38,7 +38,7 @@ RUN cp "/root/.m2/repository/com/google/api/gapic-generator-java/${DOCKER_GAPIC_
 FROM us-docker.pkg.dev/artifact-foundry-prod/docker-3p-trusted/python@sha256:afc139a0a640942491ec481ad8dda10f2c5b753f5c969393b12480155fe15a63 as final
 
 ARG OWLBOT_CLI_COMMITTISH=3a68a9c0de318784b3aefadcc502a6521b3f1bc5
-ARG PROTOC_VERSION=3.0.0
+ARG PROTOC_VERSION=21.7
 ARG GRPC_VERSION=1.76.1
 ENV HOME=/home
 ENV OS_ARCHITECTURE="linux-x86_64"