Is your feature request related to a problem? Please describe.
LfMerge has been on Mercurial 3 for a long time, and we've had constant trouble with HTTPS certificates because our Mercurial version was too old to support SNI. Moving to Mercurial 6 (and Python 3) will fix that.

As a bonus, this also moves LfMerge to .NET 8 so we could start using modern C# features in the LfMerge codebase in the future.

Describe the solution you'd like
Version 2.0.138 of the lfmerge container image updates the Mercurial version to version 6.