Update latest version of NUnit Console and Engine to 3.20

[CharliePoole]

The download page is static html, so I updated the date and last version for nunit-console.

Other downloads are also out of date. Maybe this page could be generated in some way to avoid the need to update it.

[netlify]

❌ Deploy Preview for fervent-khorana-e00513 failed.

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[CharliePoole]

@OsirisTerje I have no idea how this works, so let me know if I have to do something else. :-)

[mikkelbu]

@OsirisTerje I have no idea how this works, so let me know if I have to do something else. :-)

@CharliePoole I don't think this is related to your PR, but rather that the pipeline is not working anymore (it has been ~9 months since the last build). I don't know anything about gem dependencies nor Ruby, but I can see that @SeanKilleen corrected the last build - #158 - so he might have some idea of how to solve this

EDIT: Perhaps we can pin some of the dependencies so we don't get this bigdecimal that requires native extensions, perhaps we can pin activesupport below 8, but I've to leave now (I can try some experiments later today)