[META] Remove outdated references to "upgrade" from core

Follow-up to #2701541: Rename Drupal Upgrade UI to Migrate Drupal UI

Problem/Motivation

• "Upgrading" previously referred to using update.php to go from one major version to the next, and was distinguished from "updating" (using update.php to go from one patch/minor version to the next).
• The difference between "upgrading" and "updating" was always confusing to most people including core devs, and Migrate is a drastically different process both internally and for site owners.
• #2701541: Rename Drupal Upgrade UI to Migrate Drupal UI fixes the name of the Migrate Drupal UI module in this regard, but left behind some other uses of "Upgrade" within the module:

Proposed resolution

Find all instances of "upgrade", "upgrading", etc. Replace with consistent terminology that is more consistent when appropriate (e.g., referring to major- or minor-version updates).

Remaining tasks

TBD

User interface changes

TBD (some at least in the Migrate Drupal UI).

API changes

TBD

Data model changes

TBD