Fixing HTML in check instructions #3293

[derekashauer]

Instructions for check payments would save with < p > tags wrapping everything and show it in the admin textarea box after saving

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Changes proposed in this Pull Request:

Resolves #3293 by using esc_textarea to properly prep the text for the textarea field

How to test the changes in this Pull Request:

1. Admin settings go to Payment Gateway
2. Choose check
3. Enter instructions, save

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Fix issue saving check payment instructions adding unnecessary < p > tags in admin settings

[dparker1005]

The Pay By Check Add On currently relies on the <p> tags being added to the setting, but I just opened a PR here to update PBC to use the same output code as core, which should resolve this issue and allow us to merge this PR:
strangerstudios/pmpro-pay-by-check#138

[andrewlimaza]

This PR looks good, I ran a test to ensure HTML tags were still allowed since it's moving towards esc_textarea and not wp_kses_post. We should make the same change to Pay By Check Add On just to ensure there are no discrepancies - strangerstudios/pmpro-pay-by-check#138

I don't think people would be adding too much logic to the instructions, but keeping support for basic HTML is important.