Add missing uninstall.php to remove plugin's database option

[westonruter]

In preparing for the PL 3.0.0 release, I was looking at the build directory after running npm run build-plugins. I noticed that while webp-uploads has an uninstall.php now (#1116), and an uninstall.php is also present for optimization-detective, there is none for Speculative Loading. This adds the missing uninstall.php, adapted from the one from webp-uploads.

This also bumps the plugin version to 1.2.0 and updates the changelog.

[github-actions]

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Co-authored-by: westonruter <westonruter@git.wordpress.org>
Co-authored-by: felixarntz <flixos90@git.wordpress.org>
Co-authored-by: mukeshpanchal27 <mukesh27@git.wordpress.org>

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[felixarntz]

Thanks @westonruter. Approving, though minor nit-pick findings below.