chore: update yamlfmt to 0.21.0 (#5542)

Author: coryan

.gcb/format.yaml

@@ -98,9 +98,9 @@ steps:
       #!/usr/bin/env bash
       set -euv
       # Make three attempts to avoid download flakes.
-      go install github.com/google/yamlfmt/cmd/yamlfmt@v0.13.0 || \
-        go install github.com/google/yamlfmt/cmd/yamlfmt@v0.13.0 || \
-        go install github.com/google/yamlfmt/cmd/yamlfmt@v0.13.0
+      go install github.com/google/yamlfmt/cmd/yamlfmt@v0.21.0 || \
+        go install github.com/google/yamlfmt/cmd/yamlfmt@v0.21.0 || \
+        go install github.com/google/yamlfmt/cmd/yamlfmt@v0.21.0
       git ls-files -z -- '*.yaml' '*.yml' ':!:testdata/**' ':!:**/generated/**' ':!:librarian.yaml' | \
         xargs -0 yamlfmt
       V=$(sed -n 's/^version: *//p' librarian.yaml)

doc/contributor/howto-guide-set-up-development-environment.md

@@ -260,7 +260,7 @@ We use `yamlfmt` to format hand-crafted YAML files (mostly GitHub Actions).
 Install and use with:
 
 ```bash
-go install github.com/google/yamlfmt/cmd/yamlfmt@v0.13.0
+go install github.com/google/yamlfmt/cmd/yamlfmt@v0.21.0
 ```
 
 use with: