fix(docs): generate javadocs

[yury-s]

Javadoc options generated by the plugin have changed to include -source 8:

$ diff good.txt bad.txt 
5a6,7
> -source
> '8'
24a27,28
> 'https://docs.oracle.com/javase/8/docs/api' '/Users/yurys/playwright-java/playwright/target/javadoc-bundle-options'
> -linkoffline

The options are from the following:

Command line was: /Library/Java/JavaVirtualMachines/jdk-21.jdk/Contents/Home/bin/javadoc @options

Refer to the generated Javadoc files in '/Users/<user>/playwright-java/playwright/target/site/apidocs' dir.

Turns out -source '8' triggers an assertion in the javadoc tool, so for now we just pass 11 as the source version. Our sources are compiled with java 8 compatibility mode, so it shouldn't make any difference in the generated docs.

Reference #1533