chore(CODEOWNERS): add Go Gemini devs to ai (#10278)

Make the Go developers working on the Gemini SDK owners of the ai
component, so they can do things like approve release PRs.

Author: jba

.github/CODEOWNERS

@@ -1,6 +1,7 @@
 # Default owner for all directories not owned by others
 *                       @googleapis/yoshi-go-admins
 
+/ai/                    @googleapis/go-vertexai @googleapis/yoshi-go-admins
 /bigtable/              @googleapis/cloud-native-db-dpes @googleapis/api-bigtable-partners @googleapis/api-bigtable @googleapis/yoshi-go-admins
 /bigtable/cmd/cbt       @igorbernstein2 @googleapis/cloud-native-db-dpes @googleapis/api-bigtable-partners @googleapis/api-bigtable @googleapis/yoshi-go-admins
 /bigtable/cmd/emulator  @igorbernstein2 @googleapis/cloud-native-db-dpes @googleapis/api-bigtable-partners @googleapis/api-bigtable @googleapis/yoshi-go-admins