Use ubuntu2004 on Bazel CI (envoyproxy#19282)

gcc version is too old on ubuntu1804, which caused envoyproxy#19196

Signed-off-by: Yun Peng <[email protected]>

Author: meteorcloudy

.bazelci/presubmit.yml

@@ -2,7 +2,7 @@
 tasks:
   rbe:
     name: "RBE"
-    platform: ubuntu1804
+    platform: ubuntu2004
     test_targets:
     - "//test/common/common/..."
     - "//test/integration/..."
@@ -14,7 +14,7 @@ tasks:
     - "--jobs=75"
   coverage:
     name: "Coverage"
-    platform: ubuntu1804
+    platform: ubuntu2004
     shell_commands:
     - "sudo apt -y update && sudo apt -y install automake autotools-dev cmake libtool m4 ninja-build"
     - "wget https://apt.llvm.org/llvm.sh && sudo bash llvm.sh 10"