fix(input, select, textarea): change type of placeholder prop to string only

[willmartian]

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)
• 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 Number: resolves #22976

What is the new behavior?

• Updated the placeholder property on ion-input, ion-textarea, and ion-select to have a type of string | undefined.

Does this introduce a breaking change?

• Yes
• No

[liamdebeasi]

Let's make sure we update the BREAKING.md document with this since it's a change to the public API. You can probably add breaking change notes for ion-input and ion-textarea each similar to what we did for ion-modal and ion-popover.

[liamdebeasi]

Looks good! When you create the merge commit, be sure to add a "BREAKING CHANGE" line so it shows up in the changelog.

Example:

resolves #22976

BREAKING CHANGE: Updated the `placeholder` property on `ion-input`, `ion-textarea`, and `ion-select` to have a type of `string | undefined`.