Update RP schema for upgrades
Why is this change being made?
A new upgrade item is being added (!48711 (closed)), per the sample: https://gitlab.com/gitlab-com/www-gitlab-com/-/blob/master/data/release_posts/unreleased/samples/upgrade_notes.yml.
However, the release post item validator (https://gitlab.com/gitlab-com/www-gitlab-com/-/blob/master/bin/validate-release-post-item) rejects what appears to be a well-formed post.
This MR updates the schema for RP validation to name the Upgrade fields. This makes !48711 (closed) pass.
CC @mjang1, the TW for the RP this month, @cnorris, who's thinking about validation of RP items at the moment, @fseifoddini who is the RP manager this month.