text: Title: Collections and String: Add `ignore_case` as alias for `case_insensitive`
  Body: We added case-insensitive comparison support to various Collections keywords so that keywords got an optional `ignore_case` argument (#4343). While doing that, we noticed that four Collections keywords already supported case-insensitivity and had `case_insensitive` argument for that purpose. This kind of inconsistency within a library isn't great, so something needs to be done to it.
  
  An easy solution would be using `case_insensitive` also with the new keywords. That would make Collections consistent, but because we use `ignore_case` in BuiltIn (#2439), different libraries would be inconsistent. I believe we should aim for consistency with all standard libraries and also believe that that `ignore_case` is a better argument name than `case_insensitive`. It is also consistent with other similar arguments like `ignore_order` and `ignore_keys` in Collections.
  
  The plan is to do the following:
  - Add `ignore_case` argument to Collections keywords that accept `case_insensitive`. The old argument should be preserved for now, but it should be mentioned in the documentation that the new one is preferred. We shouldn't yet emit deprecation warnings, but we can start doing that in RF 8.0.
  - For consistency reasons, add `ignore_whitespace` to these same keywords as an alias for `whitespace_insensitive`. It should be treated the same way a `case_insensitive`.
  - For consistency reasons, add `ignore_case` as an alias also for the two String library keywords that accept `case_insensitive`.
  
  Although we won't be emitting deprecation warnings, it's good to mention this under the deprecated features section in the release notes.