fix: do not apply the_content filter to homepage post excerpts

[dkoo]

All Submissions:

• Have you followed the Newspack Contributing guideline?
• Does your code follow the WordPress' coding standards and VIP Go coding standards?
• Have you checked to ensure there aren't other open Pull Requests for the same update/change?

Changes proposed in this Pull Request:

Removes the the_content filter applied to excerpts generated for Homepage Posts blocks, since it might result in unwanted behavior if other plugins are using that same filter.

How to test the changes in this Pull Request:

1. On master, temporarily add a function to your theme's functions.php to append some content on the the_content hook:

function newspack_test_content_filter( $content ) {
	$content .= ' And this is my appended text!';
	return $content;
}
add_filter( 'the_content', newspack_test_content_filter, 0 );

2. Add a Homepage Posts block to a page and turn on "Show Excerpts". Observe that excerpts are affected by your test filter in the editor and on the front-end.
3. Check out this branch, confirm that your excerpts are unaffected by the content filter.

Other information:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes, as applicable?
• Have you successfully ran tests with your changes locally?

[laurelfulford]

I was able to recreate this issue with the provided function, both on Custom excerpts and really short excerpts (so I could see the end of them in the block). This PR fixed both cases! 🚢

[matticbot]

🎉 This PR is included in version 1.39.1 🎉

The release is available on GitHub release

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