Change the BitBucket import page to indicate that its for BB Cloud only, not for on-prem BB

Description

Our current BitBucket importer only works for BitBucket.org and does not work with on-prem/self-hosted BitBucket servers.
We need to update the docs to reflect this.

Checklist when modifying an existing page:

• What is the goal of changing the page?
• What metrics are we trying to affect?
• How will we decide if the change was successful or not?
• Would we revert the changes if we decided that it was not a success?
• Make sure events are reflected in Google Analytics/Marketo if the page is changed
• Make sure redirection is done if we change the page so email newsletters do not contain dead link

Proposal

Links / references

This is the page in question...
https://docs.gitlab.com/ee/user/project/import/bitbucket.html

There is an issue for an on-prem BitBucket server importer here...
https://gitlab.com/gitlab-org/gitlab-ee/issues/1856