Finalize migration requirement for stable database semconv

[trask]

Fixes #719

Changes

Finalizes the migration requirement for instrumentations to follow when updating to stable database semconv.

[pyohannes]

Warning
Existing Database instrumentations that are using
v1.20.0 of this document
(or prior):

• SHOULD NOT change the version of the networking conventions that they emit
  until the HTTP semantic conventions are marked stable (HTTP stabilization will
  include stabilization of a core set of networking conventions which are also used
  in Database instrumentations).

Does it makes sense that this part still coexists with the warning proposed in this PR?

If I understand correctly, this would require users to set both OTEL_SEMCONV_STABILITY_OPT_IN=database,http to reliably get data according to new (stable) conventions?

[gregkalapos]

nit - somewhat related to @pyohannes's comment. In the http docs in the part about OTEL_SEMCONV_STABILITY_OPT_IN it currently says:

The only values defined so far are:

If these new values are added due to DB, I think that sentence should be removed from the http docs.

[trask]

Does it makes sense that this part still coexists with the warning proposed in this PR?

good point, it does make it much more confusing

I've removed the http part. That part still exists in prior tags, so still can apply if someone is upgrading to earlier version of semconv.