Install wkhtmltopdf 12.6.0 on Ubuntu 20.04 LTS (focal)

For Ubuntu 20.04 LTS (focal), install wkhtmltopdf 12.6.0 (see https://wkhtmltopdf.org). The script downloads the binary from the official GitHub repository, installs it and then deletes the original downloaded file. If everything has worked when running "wkhtmltopdf --version" gives the output "wkhtmltopdf 0.12.6 (with patched qt)". To run a quick test yourself run "wkhtmltopdf https://laravel.om laravel.pdf", to generate a PDF of the Laravel homepage.