Fix truncated merge tags in HTML attribute when included in PDF setting Rich Text fields

[jakejackson1]

Description

Only field merge tags were being matched during rich text sanitizing. This adjustment now matches any merge tag with opening and closing {}, provided it is the only content included in a HTML attribute.

Testing instructions

1. Add <img src="{user:user_profile_picture}" /> to Footer HTML field
2. Save PDF settings
3. Verify the merge tag hasn't been truncated

Checklist:

• [x] I've tested the code.
• [x] My code is easy to read, follow, and understand
• [ ] My code follows the accessibility standards.
• [x] My code has proper inline documentation / docblocks.

Additional Comments

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## hot-patch-6.11.2 #1562 +/- ## =================================================== Coverage ? 76.05% =================================================== Files ? 248 Lines ? 12999 Branches ? 370 =================================================== Hits ? 9887 Misses ? 3104 Partials ? 8 ```

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