jvm.optionsand startup.options values should have sensible defaults in code

[robbavey]

Since #13825, we have added the ability to have mandatory JVM options defined in code, rather than simply in jvm.options.

This helps ensure that when Logstash users perform upgrades, which keep modified jvm.options in tact after an upgrade, their Logstash installation remains in a working state even with their modified jvm.options.

We should extend this to other jvm.options (and startup.options) values, and have sensible defaults defined in code. This would treat jvm.options and startup.options similarly to logstash.yml, where the defaults are defined in code, with the contents of those files used to "document" common overrides. This gives us the opportunity to update and add defaults when appropriate for all users, and not missing those performing upgrades.

Related: #16773, #15925