Issue #3576835: Preserve columns when enabling formatted text

Closes #3576835

Formatted text elements were selectable in existing handlers, but version 2.1.1 omitted their values from exported rows. Enabling support would otherwise add a new cell and shift every following value.

This post-update excludes single-value formatted text elements from existing Google Sheets handlers. Administrators can select them again after adding the corresponding spreadsheet columns. New handlers are unchanged.

Test coverage verifies that formatted text is excluded while ordinary text fields remain selected.

Testing:

• ddev phpcs webform_googlesheets.post_update.php tests/src/Kernel/WebformGoogleSheetsPostUpdateTest.php
• ddev phpunit --filter WebformGoogleSheetsPostUpdateTest