Add deprecation warning for the existing teams feature

[gtsiolis]

Problem to solve

The upcoming Teams & Projects feature is going to be released in the next days. We're also working on some Plans & Pricing updates which will based on Teams & Projects, to be released later this year.

However, we're still using the term Team within the existing Teams feature, which could confuse our users. See relevant docs.

See also relevant discussion (internal).

Proposal

1. A good MVC (minimum viable change) could be to add an information alert to help avoid user confusion and increase feature awareness. We recently added a similar information alert for the Theia editor deprecation in [dashboard] Add Theia deprecation warning #4614.
2. Adding a deprecation notice in the relevant docs could also help.
3. Optionally, we could consider renaming the existing Teams feature to Subscriptions.

[gtsiolis]

Closing this, we went with option 3 and renamed the feature.

FWIW, we decided to go with Team Plans instead of Subscriptions, see #6011. Cc @gitpod-io/engineering-meta