Update app.yaml

go 1.15 support ends soon. https://cloud.google.com/appengine/docs/standard/lifecycle/support-schedule

Need to actually test the deploy and make sure this doesn't break anything.

site/app.yaml

@@ -12,7 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-runtime: go115
+runtime: go121
 service: development
 
 handlers: