Docs: Specify the minimum version of Bitbucket Server expected

New doc or update?

• New documentation
• Update existing documentation

Checklists

Product Manager

• Add the correct labels
• Add the correct milestone
• Indicate the correct document/directory for this feature
• Fill the doc blurb below

Documentation

Add to documentation a minimum Bitbucket Server version we expect, 5.5+ has been noted in the page comments and initially confirmed.

/cc @stanhu @jameslopez @cwest1