Rename persisted plan titles

Summary

EoA has rolled out, but there are instances of Silver, Gold, etc that remain in GitLab

Steps to reproduce

Purchase a new post-EoA license(eg. Premium, Ultimate) for a group.
Once purchased, as an admin, go to Admin > Groups and select that group.

What is the current bug behavior?

If a premium license was purchase, then the mention Silver will show up.

What is the expected correct behavior?

This mention should be updated to Premium (Formerly Silver) (in order to be consistent with what we see on the group billing page).

Relevant logs and/or screenshots

Output of checks

Results of GitLab environment info

Expand for output related to GitLab environment info


(For installations with omnibus-gitlab package run and paste the output of:
`sudo gitlab-rake gitlab:env:info`)

(For installations from source run and paste the output of:
`sudo -u git -H bundle exec rake gitlab:env:info RAILS_ENV=production`)

Results of GitLab application Check

Expand for output related to the GitLab application check

(For installations with omnibus-gitlab package run and paste the output of:
sudo gitlab-rake gitlab:check SANITIZE=true)
(For installations from source run and paste the output of:
sudo -u git -H bundle exec rake gitlab:check RAILS_ENV=production SANITIZE=true)
(we will only investigate if the tests are passing)