fix(item): form validation caret color renders correctly

[liamdebeasi]

Pull request checklist

Please check if your PR fulfills the following requirements:

• [ ] Tests for the changes have been added (for bug fixes / features)
• [ ] Docs have been reviewed and added / updated if needed (for bug fixes / features)
  • Some docs updates need to be made in the ionic-docs repo, in a separate PR. See the contributing guide for details.
• [ ] Build (npm run build) was run locally and any changes were pushed
• [ ] Lint (npm run lint) has passed locally and any fixes were made for failures

Pull request type

Please check the type of change your PR introduces:

• [ ] Bugfix
• [ ] Feature
• [ ] Code style update (formatting, renaming)
• [ ] Refactoring (no functional changes, no api changes)
• [ ] Build related changes
• [ ] Documentation content changes
• [ ] Other (please describe):

What is the current behavior?

Issue URL: resolves https://github.com/ionic-team/ionic-framework/issues/25719

When simplifying the code for the input highlights/caret color in https://github.com/ionic-team/ionic-framework/commit/5ca2ce91971408218d7bdc52509ce61a6ebb46aa, I accidentally made the caret color apply to both iOS and MD modes. This behavior was added to align with the MD spec (https://material.io/components/text-fields), so it should only be available on MD mode.

Additionally, the usage of --highlight-color-focused was incorrect. We should use --highlight-background as the value of that will be set to --highlight-color-focused when the input is focused. In other words, we don't need to reference the var directly.

What is the new behavior?

• Caret color correction applies on MD mode only
• Removed the unnecessary highlight-color-focused reference. Note: There should be no noticeable change here because the code in item.scss was always overriding the incorrect code in item.md.scss.

Does this introduce a breaking change?

• [ ] Yes
• [x] No

Other information