Fix 'Trying to access array offset on value of type bool' in WP 5.0-5.2

[IanDelMar]

Description

This PR adds true as the second argument to add_theme_support( 'post-thumbails' ).

Motivation and Context

Prior to version 5.3 calling add_theme_support( 'post-thumbails' ) without the second argument makes WP try to access the index of a variable with value true. PHP throws a 'Trying to access array offset on value of type bool' notice. As Understrap currently supports WP versions back to version 5.0 we should fix this.

See https://github.com/WordPress/WordPress/blob/d1e96dfce29333740f8a5042e2e760746ede89ea/wp-includes/theme.php#L2340-L2362 (WP 5.2.15) for the code causing the notice and see https://github.com/WordPress/WordPress/blob/6d40400756c0dc14c7f9aac1e60f6a84fc428b3d/wp-includes/theme.php#L2385-L2405 (WP 5.3) for the code fixing the notice.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• I pulled my branch from develop.
• I am submitting my pull request to develop.
• I have resolved any conflicts merging this pull request would create.
• I have checked that there aren't other open Pull Requests for the same update/change.
• My code follows the code style of this project.
• I have performed a self-review of my own code.
• I have commented my code, particularly in hard-to-understand areas.
• (Optional) My change requires a change to the documentation.
• (Optional) I have updated the documentation accordingly.
• (Optional) My change requires a change to the translations.
• (Optional) I have updated the translations accordingly.
• composer cs:check has passed locally.
• composer lint:php has passed locally.
• I have read the CONTRIBUTING document.

Related Issues or Roadmap requests

Further comments

[IanDelMar]

@bacoords safe to merge