1.21 was patched by 1.21.1, and 1.21.2 was patched by 1.21.3, so the original NMS versions aren't available anymore. This was correctly updated in `commandapi-bukkit`, but not in `commandapi-spigot`. `commandapi-spigot` should now match `commandapi-bukkit`.

On `dev/dev`, `ConfigurationAdapter#saveDefaultConfig` could be used to save the default settings for a platform to a file. However, since Spigot and Paper have different config options, `DefaultBukkitConfig` was updated to handle both Paper and Spigot configs. Therefore, `BukkitConfigurationAdapter#saveDefaultConfig` did not have a single default config, so the signature of `saveDefaultConfig` was updated to input the instance of `DefaultConfig` that should be written to the file.

This commit fixes a mistake where Velocity was still trying to use the `saveDefaultConfig` method with only two arguments. Since `saveDefaultConfig` no longer always uses *the* default config for the platform, I also renamed the method to `saveConfig`.

Something to think about in code review: `DefaultBukkitConfig` is in `commandapi-bukkit`, but it contains the Paper-specific field `SHOULD_HOOK_PAPER_RELOAD`. This field does not need to exist on Spigot, so should it be moved into a Paper-specific subclass of `DefaultBukkitConfig`, similar to what happens with `CommandAPIBukkitConfig` and `CommandAPIPaperConfig`?
Also, do we need 4 `CommandAPIMain` classes? Can we put them into a common module like before? Perhaps the command conversion logic can be extracted into the Bukkit module, and the config stuff can be extracted into a `spigot/paper-plugin-common`

Makes it easier to distinguish test cases versus classes that help the tests work (e.g. `TestBase`, `MockCommandAPIServer`, etc.)

Created Mock versions for classes that are meant to be implemented in Paper/Spigot.
Select NMS method implementations are now overridden using a Mockito spy rather than manually reimplementing all methods and directing some to the original object.
Made some changes and comments in non-test classes based on test results.

TODO:
- Make Paper/Spigot specific tests for chat arguments
- Fix CustomArgument tests
- Update test-impl for other versions