Only warn about compatibility dates on publish

[Electroid]

Changes the compatibility date warning to only occur when using wrangler publish. We do not want this warning to be sent when using other commands like wrangler tail, etc.

Closes #2078

[threepointone]

• We should probably have this for dev too, or you risk getting divergent behaviour.
• This should probably be an error on publish, not a warning (or folks who don't manually publish won't even see it)

[Electroid]

Hm, I don't think we should error on publish. While this calls for action, it's not urgent or important enough to break builds. If we were deprecated something that might be one thing, but this will continue to work. I think a warning is fine.

[ocsfrank]

Looks good to me! I also think just a warning should be fine.

[threepointone]

Could you add the warning for dev as well? Let's merge this in and release it right after.