Exception calling IAM: There were concurrent policy changes - gcloud

Question

I get the above error after running mvn -DskipTests package appengine:deploy. Here's the whole error:
Exception calling IAM: There were concurrent policy changes. Please retry the whole read-modify-write with exponential backoff. The request's ETag '\007\005\364\274\033.\262\307' did not match the current policy's ETag '\007\005\364\274\033\177\220\315'.
From my reading, this occurs when there multiple policy changes at the same time. The other questions I've seen throw this error in regard to a POST request but this is to do with an appengine:deploy
What IAM policy does the service account need to work?
This is the IAM account that I believe was created by the appengine:deploy process and it's permissions:
Note - I don't think it's applicable but I added this deploy to appengine step to a Spring Boot app speakiong to a Postgres db.
EDIT
As mentioned in the comment, #Robina's answer has moved me along but not the logs show this error:
UnsupportedClassVersionError: org/springframework/boot/loader/JarLauncher has been compiled by a more recent version of the Java Runtime (class file version 61.0), this version of the Java Runtome only recognizes class file version up to 55.0
But setting the java version to 17 throws a build error; here's my appengine.yaml:
runtime: java11
instance_class: F1
EDIT 2
I came across this while reading through the Google guide here which said:
Deploying an executable JAR
Use any build framework to build an executable JAR locally, then do one of the following depending on whether you created an app.yaml file for your app:
If you created an app.yaml file:
Copy the file into the same directory as the executable JAR file you created.
From the directory that contains the app.yaml and your JAR, enter the following command:
gcloud app deploy
gcloud app deploy will create an app.yaml file that contains the
minimum settings, using all default values.
I thought, why not? So I deleted the appengine folder and its app.yaml and then ran the command from above, minus the jar:
gcloud app deploy
After, probably 10 minutes of the command running the app was deployed and points to the Postgres database without any problems. Thanks #Robina for the help!

Answer 1

The service account must have the App Engine Deployer role in order to deploy an application to App Engine. The IAM policy needed for the service account to function depends on what it is used for. Your application is probably being deployed to Google App Engine using the service account if you are using appengine:deploy.
A gcloud command example for granting the role:
gcloud projects add-iam-policy-binding [PROJECT_ID] --member=serviceAccount:[SERVICE_ACCOUNT_EMAIL] --role=roles/appengine.deployer
[PROJECT ID] and [SERVICE ACCOUNT EMAIL] should be changed to reflect your project ID and service account email, respectively.