Fix production marketplace URL
We changed the production marketplace URL to api.integrations.mattermost.com, but I never backported https://github.com/mattermost/mattermost-server/pull/12378 to v5.16, and we shipped with the invalid marketplace.integrations.mattermost.com instead.
has configured a second hostname to keep the feature working for v5.16 customers who migrate today, but I'd like to patch this earlier rather than later to allow us to converge back onto a single endpoint at some point in the future.
QA Test Steps
Spin up a new test server (no existing config.json). Verify plugin marketplace url is set to `https://api.integrations.mattermost.com`
Migrate a v5.16 (pre-fix) server with an existing `config.json` that has the old `https://marketplace.integrations.mattermost.com`. Upon upgrade to the patch, verify the plugin marketplace url is set to `https://api.integrations.mattermost.com`
Tested and verified during PR review (PR test server) as well as in local and Jenkins upgrade test servers (5.16 to 5.16-rc2).
The marketplace URL correctly points to api.integrations.mattermost.com.