chore(internal/gapicgen): update gapic generator v0.15.3 (#3063)
chore(internal/gapicgen): update gapic generator v0.15.3 (#3063)

Updates `gapic-generator-go` to `v0.15.3`. Notable changes include:

* use DefaultEndpoint and DefaultMTLSEndpoint for default options (https://github.com/googleapis/gapic-generator-go/pull/465)
googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go