Fix: builds for Unity versions less than 2019 fail

[cabauman]

Changes

Discovered when the gameci/docker repo CI failed in this PR.

Assets/Editor/Editor/UnityBuilderAction/Input/AndroidSettings.cs(14,32): error CS0117: 'PlayerSettings.Android' does not contain a definition for 'useCustomKeystore'

It's complaining about the line added in this unity-builder commit on May 21st.

PlayerSettings.Android.useCustomKeystore = true;

useCustomKeystore was added in Unity 2019 which is why 2018 builds fail.

Might also be worth adding Unity 2018 to the CI version list to prevent regressions.

Checklist

• Read the contribution guide and accept the code of conduct
• Readme (updated or not needed)
• Tests (added, updated or not needed)

[github-actions]

[codecov-commenter]

Codecov Report

Merging #467 (1ab54fe) into main (f054aaa) will not change coverage.
The diff coverage is n/a.

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##             main     #467   +/-   ##
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  Coverage   52.52%   52.52%           
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  Files          66       66           
  Lines        2056     2056           
  Branches      353      385   +32     
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  Hits         1080     1080           
  Misses        972      972           
  Partials        4        4