Fixes #1864

Related PRs:
gutenberg-pr: WordPress/gutenberg#20095

To test:

Blocks with PlainText:

• code
• shortcode

Existing Field

1. Open a page or post with a block that uses the PlainText component.
2. Click on the field
3. Edit or leave unchanged
4. Click the keyboard dismiss button
   Expect the keyboard to dismiss

New Field

1. Select to add a new component with the PlainText component
   Expect the keyboard to dismiss
2. Add text or leave blank
3. Click the keyboard dismiss button
   Expect the keyboard to dismiss

PR submission checklist:

• I have considered adding unit tests where possible.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.