* Enable automatic upgrade server by default
This commit means the proxy now serves automatic upgrade channels by
default. The default channel is "default" and its version is the
teleport proxy binary version.
This commit also fixes a bug with the automatic update version server
returning `teleport.Version` instead of `"v"+teleport.Version` when the
version was too high. This was not a critical issue because agents are
supposed to tolerate versions not prefixed with 'v'.
* lint
* thread safety
* Update lib/automaticupgrades/channel.go
* Update lib/config/configuration.go
Co-authored-by: Noah Stride <noah.stride@goteleport.com>
* fix tests and lint
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Co-authored-by: Noah Stride <noah.stride@goteleport.com>