Changelog: fix style to make it consistent with the rest of the file

Merged as part of !2721 (merged)

Context

This MR fixes the style of the items that were added to the changelog to make it consistent with the rest of the file (in particular: use past tense).

WARNING: this MR assumes that !2704 (merged) will be merged.

Manually testing the MR

Read the changelog.

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, the Development Version section of CHANGES.md for everything else).
• Select suitable reviewers using the Reviewers field below.