Support newer versions of REAPI

[peterebden]

Not actually that much we need to do here. Overview:

• Handling newer symlink fields correctly (this is not well supported by remote-apis-sdks).
• We now negotiate a version within the range [2.1, 2.12] rather than a single version.
• Don't crash if the server returns nil for the version fields
• Use the deprecated server version as the true minumum, with a warning
• Fix a minor issue where we were inconsistent about platform properties between Command and Action

On versioning: 2.1 is the minimum we truly require although there are some clarified PATH semantics in 2.3 that are somewhat relevant. They can be configured with the remote.shell config property to make it work, so we shouldn't just fail a 2.1 server, and the clarification was basically codifying retroactively what servers already did in the wild anyway.
Sticking with 2.1 as a minimum should avoid any issues with us upgrading internally; also AFAIK BuildGrid and JustBuild report 2.2 and 2.1 as a max version.

Also includes #3612

I probably won't land this until I'm sure current master is stable; I am pretty sure it's all safe but one thing at a time.