Fix gcc versions in platform labels (#1447)

Some of the most recent Docker images contained gcc upgrades.
diff --git a/buildkite/bazelci.py b/buildkite/bazelci.py
index e64386f..da9483f 100755
--- a/buildkite/bazelci.py
+++ b/buildkite/bazelci.py
@@ -512,24 +512,24 @@
         "python": "python3.6",
     },
     "ubuntu1804": {
-        "name": "Ubuntu 18.04 LTS (OpenJDK 11, gcc 7.4.0)",
-        "emoji-name": ":ubuntu: 18.04 LTS (OpenJDK 11, gcc 7.4.0)",
+        "name": "Ubuntu 18.04 LTS (OpenJDK 11, gcc 7.5.0)",
+        "emoji-name": ":ubuntu: 18.04 LTS (OpenJDK 11, gcc 7.5.0)",
         "downstream-root": "/var/lib/buildkite-agent/builds/${BUILDKITE_AGENT_NAME}/${BUILDKITE_ORGANIZATION_SLUG}-downstream-projects",
         "publish_binary": ["ubuntu1804"],
         "docker-image": f"gcr.io/{DOCKER_REGISTRY_PREFIX}/ubuntu1804-java11",
         "python": "python3.6",
     },
     "ubuntu2004": {
-        "name": "Ubuntu 20.04 LTS (OpenJDK 11, gcc 9.3.0)",
-        "emoji-name": ":ubuntu: 20.04 LTS (OpenJDK 11, gcc 9.3.0)",
+        "name": "Ubuntu 20.04 LTS (OpenJDK 11, gcc 9.4.0)",
+        "emoji-name": ":ubuntu: 20.04 LTS (OpenJDK 11, gcc 9.4.0)",
         "downstream-root": "/var/lib/buildkite-agent/builds/${BUILDKITE_AGENT_NAME}/${BUILDKITE_ORGANIZATION_SLUG}-downstream-projects",
         "publish_binary": [],
         "docker-image": f"gcr.io/{DOCKER_REGISTRY_PREFIX}/ubuntu2004-java11",
         "python": "python3.8",
     },
     "ubuntu2004_arm64": {
-        "name": "Ubuntu 20.04 LTS ARM64 (OpenJDK 11, gcc 9.3.0)",
-        "emoji-name": ":ubuntu: 20.04 LTS ARM64 (OpenJDK 11, gcc 9.3.0)",
+        "name": "Ubuntu 20.04 LTS ARM64 (OpenJDK 11, gcc 9.4.0)",
+        "emoji-name": ":ubuntu: 20.04 LTS ARM64 (OpenJDK 11, gcc 9.4.0)",
         "downstream-root": "/var/lib/buildkite-agent/builds/${BUILDKITE_AGENT_NAME}/${BUILDKITE_ORGANIZATION_SLUG}-downstream-projects",
         "publish_binary": [],
         "docker-image": f"gcr.io/{DOCKER_REGISTRY_PREFIX}/ubuntu2004-java11",
@@ -539,8 +539,8 @@
         "always-pull": False,
     },
     "kythe_ubuntu2004": {
-        "name": "Kythe (Ubuntu 20.04 LTS, OpenJDK 11, gcc 9.3.0)",
-        "emoji-name": "Kythe (:ubuntu: 20.04 LTS, OpenJDK 11, gcc 9.3.0)",
+        "name": "Kythe (Ubuntu 20.04 LTS, OpenJDK 11, gcc 9.4.0)",
+        "emoji-name": "Kythe (:ubuntu: 20.04 LTS, OpenJDK 11, gcc 9.4.0)",
         "downstream-root": "/var/lib/buildkite-agent/builds/${BUILDKITE_AGENT_NAME}/${BUILDKITE_ORGANIZATION_SLUG}-downstream-projects",
         "publish_binary": [],
         "docker-image": f"gcr.io/{DOCKER_REGISTRY_PREFIX}/ubuntu2004-java11-kythe",