Drop 'indentSize'

[mateuszkwiecinski]

From the discussion in pinterest/ktlint#1434 (comment) I understood overriding an .editorConfig property via userData parameter won't be supported.
According to pinterest/ktlint#1442 passing indent_size will result in a runtime exception. Regardless if this lands in current form, in context of the discussion from previous PR - I'm proposing to remove the indentSize property from Kotlinter's extension.
Regardless of how the future of ktlint will look like, I think such change makes sense:
a) .editorconfig should be the preferred way of setting the indent_size parameter, mainly because it integrates with IntelliJ based IDEs.
b) it's less code to maintain 😛

The recommended migration path which probably should be mentioned in release notes is to replace kotlinter.indentSize with the following entry in {project_root}/.editorconfig:

[*.{kt, kts}]
indent_size = 4

[jeremymailen]

I agree completely. The indentSize in the gradle extension is vestigial at this point.
The documentation will be a little ahead of the release, but it's ok I think since we'll get the upgrade done this week.