Fix error savings forms in Gravity Forms 2.6

[jakejackson1]

Description

The \GFFormsModel::get_form_cache_key( $form_id ) method wasn't introduced until v2.7, and this was causing a fatal error in Gravity Forms 2.6 when saving the form in the editor.

Checklist:

• [x] I've tested the code.
• [ ] 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

Attention: Patch coverage is 0% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 77.03%. Comparing base (c069c6e) to head (551020e).

Files	Patch %	Lines
src/Controller/Controller_Form_Settings.php	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## development #1497 +/- ## =============================================== + Coverage 77.02% 77.03% +0.01% =============================================== Files 243 243 Lines 12449 12447 -2 Branches 370 370 =============================================== Hits 9589 9589 + Misses 2852 2850 -2 Partials 8 8 ```

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