* docs - add information on upgrading to PostGIS 2.5.4

Upgrade instructions 2.1.5 to different versions of 2.5.4

* docs - upgrade to PostGIS 2.5.4 review comments

* docs - more review comment updates.
reorder upgrade sections.
clarify removing PostGIS package, is for removing the gppkg

* docs - minor edit

* docs - review updates - more emphasis on removing PostGIS from a database deleting objects.
-Create separate paragraph in Upgrading section.
-Add warning in Removing PostGIS section

* docs - minor review comment update

* small edits
Co-authored-by: NDavid Yozie <dyozie@pivotal.io>