New features

A new setting allows you to specify the preferred root authority. On January 11th Let's Encrypt will switch over to their own root certifiticate which is not trusted by older Android versions and perhaps other (older) software. As a fallback, until September 30th it will still be possible to get certificates using the old root. In settings.json you can configure Acme.PreferredIssuer to be "ISRG Root X1" if you want to start testing with the new root today or "DST Root CA X3" to keep using the fallback as long as it will last.
A DNS validation plugin for Dutch hosting company TransIP is now available from the releases page. Note that this provider is not very fast updating its records after their API has accepted the changes, so it's highly recommended to roughly double either PreValidateDnsRetryCount and/or PreValidateDnsRetryInterval in settings.json.

Enhancements

The program is now built on .NET 5.0 instead of .NET Core 3.1. This should not have much user impact, but allows us to keep up to date with the latest Microsoft technologies and should solve some annoying issues like certain startup problems like #1632 reported by @MarcoMiltenburg.
The program will now refuse to start when it detects that another instance on the same machine is already working on the same configuration path. A warning will be logged when it detects that another instance is running for a different configuration path. Running multiple instances in parallel can cause issues in certain scenarios, for example when both try to make changes to IIS at the same time.
It's now possible to use plugins when using win-acme as a dotnet tool. To use them they need to be unpacked to %userprofile%\.dotnet\tools\.store\win-acme\{version}\win-acme\{version}\tools\net5.0\any. We realize this is not the most user friendly experience and might come up with better solutions in the future. Requested by @rprouse in #1691.
The path to the program used for a newly created scheduled task is now quoted when necessary, reported by @Phil-G in #1704.
An example PowerShell script to use win-acme for the Windows Admin Center was submitted by Matthew Barreiro, thanks!

#1706 - Crash fix for the DigitalOcean plugin when using domain substitution for the acme-challenge subdomain. Thanks for the contribution @Skulblaka.
#1700 - It was not possible to use TLS-ALPN-01 validation from the command line, reported by @andrianovSupplerus.