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:

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

Additional Comments

[codecov]

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 ⚠️

Additional details and impacted files

@@               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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