App Engine Standard should always use the uberjar

[loicmathieu]

Fixes #21564

[loicmathieu]

/cc @mkouba

[asodja]

Sorry if this if it is somehow unrelated, but I saw this change:
I was actually using AppEngine Standard Java 11 with a fast-jar. I used gcloud to deploy artifacts and I had to also specify a custom entry point.

But it might be true that if you use some maven-plugin for deploy it won't work.

[loicmathieu]

@asodja yes, this is not the only way to do it but it's the simplier way that works in all cases.

Feel free to enhance the guide with fast-jar (aka default packaging) and a default entry point.

[loicmathieu]

@mkouba did you check the wording or should I ask someone to do it ?
I don't know your level of english but mine deserve someone to check it ;)

[loicmathieu]

@geoand I applied your suggestions