[various] Update Java compatibility version to 11

[stuartmorgan]

Now that stable requires AGP 7.0, which in turn requires Java 11+, we should be able to set our compatibility version to 11 (setting the min Flutter SDK version to current stable).

Also removes the gradle conditionals for setting namespace (and the repo tool checks that it is present), as that was only needed for AGP 4.1 compatibility.

Part of flutter/flutter#156111

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