updated instructions for major buildkit version jumps

[joshdentremont]

Purpose / why

As discussed in the tech call, we wanted to make it clear that it is ok to update isle buildkit to the newest version skipping past several versions in the process.

What changes were made?

Added notes about jumping past several versions.
Added the major breaking changes in versions 4, 5, and 6 that need to be addressed if updating from earlier versions

Interested Parties

• @Islandora/documentation
• @Islandora/committers

Checklist

Pull-request Reviewer

Pull-request reviewer should ensure the following:

• Does this PR link to related issues?
• Does the proposed documentation align with the Islandora Documentation Style Guide?
• Are the changes accurate, useful, free of typos, etc?

Person Merging

The person merging should ensure the following:

• Does mkdocs still build successfully? (This is indicated by TravisCI passing. To test locally, and see warnings, see How To Build Documentation.)
• If pages are renamed or removed, have all internal links to those pages been fixed?
• If pages are added, have they been linked to or placed in the menu?
• Did the PR receive at least one approval from a committer, and all issues raised have been addressed?