Update mPDF cURL User Agent

[jakejackson1]

Description

Use the latest version of Firefox. Siteground recently started blocking requests that used very old browser versions / user agents. This stopped Gravity PDF loading images / CSS via HTTP/HTTPS.

Fixes #1440

Testing instructions

Add a Background Image to a PDF that points to a Siteground-hosted image and verify it displays correctly.

Screenshots

Checklist:

• [X] 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[bot]]

Codecov Report

Merging #1443 (f7f5a59) into development (b9671fd) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@              Coverage Diff               @@
##             development    #1443   +/-   ##
==============================================
  Coverage          75.70%   75.70%           
  Complexity          2935     2935           
==============================================
  Files                243      243           
  Lines              12826    12827    +1     
  Branches             370      370           
==============================================
+ Hits                9710     9711    +1     
  Misses              3108     3108           
  Partials               8        8           

Impacted Files	Coverage Δ
src/Helper/Helper_PDF.php	81.81% <100.00%> (+0.06%)	:arrow_up:

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