[Java] Add triple-mustache to all instances of vendorExtensions.extraAnnotation

[ThatJoeMoore]

PR checklist

• Read the contribution guildelines.
• Ran the shell/batch script under ./bin/ to update Petstore sample so that CIs can verify the change. (For instance, only need to run ./bin/{LANG}-petstore.sh and ./bin/security/{LANG}-petstore.sh if updating the {LANG} (e.g. php, ruby, python, etc) code generator or {LANG} client's mustache templates)
• Filed the PR against the correct branch: master for non-breaking changes and 2.3.0 branch for breaking (non-backward compatible) changes.

Description of the PR

This covers the usages that weren't fixed in #3825

Triple mustache is required because annotations may contain chars like
"=" that would be mistakenly encoded - see #3754

Notes

• Updating the samples didn't change anything in the servers. There were a bunch of unrelated changes to the client samples that belong in a separate commit/PR (mostly the addition of Capitalization.java)
• I'm almost certain that the changes to Mustache documented in Java: Latest JMustache breaks bigDecimalAsString #3754 will have broken other generators, but I don't even know where to begin looking for them.

[wing328]

@ThatJoeMoore thanks for the PR. The change looks good to me.