Download Zstandard bundles on GHES too by default

[henrymercer]

We're seeing an average 2x speed up by using bundles compressed using Zstandard. Now that this feature has stabilised, this PR rolls it out to GHES, cleans up the feature flags, and removes a mechanism to fall back to gzip that has not been needed.

Merge / deployment checklist

• Confirm this change is backwards compatible with existing workflows.
• Confirm the readme has been updated if necessary.
• Confirm the changelog has been updated if necessary.

[aeisenberg]

With this PR, users will no longer be able to control if they use zstd or tar. I think that's fine since it is not relevant for them if everything works anyway.

[henrymercer]

Yeah agreed. Customers can still specify a tools URL if they want to use gzip or zstd. In general I think we want to have flexibility to change the default way the tools are downloaded and installed, e.g. so we can move to per-language bundles.