dstack Improve CLI messages on new available dstack versions

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.

Apr 11 '24 16:04 jvstme

This issue is stale because it has been open for 30 days with no activity.

May 12 '24 01:05 peterschmidt85

This issue was closed because it has been inactive for 14 days since being marked as stale. Please reopen the issue if it is still relevant.

May 26 '24 01:05 peterschmidt85

Relevant

Jun 12 '24 10:06 jvstme

This issue is stale because it has been open for 30 days with no activity.

Jul 13 '24 01:07 peterschmidt85

This issue was closed because it has been inactive for 14 days since being marked as stale. Please reopen the issue if it is still relevant.

Jul 28 '24 01:07 peterschmidt85