Improve CLI messages on new available dstack versions

[jvstme]

Current

If a newer dstack version is available, CLI shows a message about it when running dstack server. The message is the same for stable and pre-release versions.

A new version of dstack is available: 0.18.0rc3

Proposed

• If a newer stable version is available, show this message:

  A new version of dstack is available: X.Y.Z
  

• If a newer pre-release version is available, show this message (or similar):

  Would you like to help us test the upcoming features and improvements in dstack? A new pre-release version is available: X.Y.ZrcN
  

• If both a newer stable and a newer pre-release version are available, only show the message about the stable version:

  A new version of dstack is available: X.Y.Z
  

Rationale

• Communicate clearly that pre-release versions are intended for testing only.
• Do not lose the reminder about a newer stable version because of a pre-release version. The user might not want to update to a pre-release version, but they should still know about the newer stable one.