deps: update docker.io/library/alpine docker tag to v3.21.0

Author: renovate-bot

.cloudbuild/library_generation/library_generation.Dockerfile

@@ -27,7 +27,7 @@ RUN mvn install -B -ntp -DskipTests -Dclirr.skip -Dcheckstyle.skip
 RUN cp "/root/.m2/repository/com/google/api/gapic-generator-java/${DOCKER_GAPIC_GENERATOR_VERSION}/gapic-generator-java-${DOCKER_GAPIC_GENERATOR_VERSION}.jar" \
   "./gapic-generator-java.jar"
 
-FROM docker.io/library/alpine:3.20.3@sha256:1e42bbe2508154c9126d48c2b8a75420c3544343bf86fd041fb7527e017a4b4a as glibc-compat
+FROM docker.io/library/alpine:3.21.0@sha256:21dc6063fd678b478f57c0e13f47560d0ea4eeba26dfc947b2a4f81f686b9f45 as glibc-compat
 
 RUN apk add git sudo
 # This SHA is the latest known-to-work version of this binary compatibility tool