exchange: use v2 bundles for modern compression engines (issue5506) Previously, `hg bundle zstd` on a non-generaldelta repo would attempt to use a v1 bundle. This would fail because zstd is not supported on v1 bundles. This patch changes the behavior to automatically use a v2 bundle when the user explicitly requests a bundlespec that is a compression engine not supported on v1. If the bundlespec is <engine>-v1, it is still explicitly rejected because that request cannot be fulfilled.