Remove remainder of Silver and Gold references in docs

Problem to solve

After the recent tier renaming of Core, Silver, and Gold to Free, Premium, and Ultimate, respectively, we still have some in-content references to these tier names that need to be changed to their new values.

This issue will attempt to replace and fix these now deprecated, remaining tier name references with their new values.

This effort is separate from removing any Starter or Bronze tiers (which are now not being sold), but may include some changes of opportunity related to them (as they need to be gotten rid of, anyways).

Procedure

This space will be used for references to tier names, by type. There may be some overlap, but I'm not worried about duplication, as I want to be thorough.

Be sure to create separate MRs for the work. We can combine some of the page changes (possibly by stage or group), but I want us to right-size any MRs we create. Also, remember to link the MRs to this issue.

Core

Silver

Gold

Other links/references

technical-writing#344

Edited by Craig Norris