* Use default upgrade channel
This commit:
- initializes default upgrade channels based on the server features
- makes all integrations use the upgrade channels instead of hitting
hardcoded s3 bucket
- makes the version channel return its own version if the target
version is too high
- makes the NoVersion handler properly: returned as an error. This way
soneone relying on the version getter doesn't have to check
- moves the version kube-agent-updater lib in main teleport libs
- add tests for noVersion channels
* Update lib/web/join_tokens.go
Co-authored-by: Bernard Kim <[email protected]>
* address marco's feedback
* address marco's feedback pt.2
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Co-authored-by: Bernard Kim <[email protected]>
