Resolve "Notes are persisted with the user's locale"

What does this MR do?

When we post a system note about assignee changes, we use to_sentence, which uses (by default) the user's locale. We don't want that, as system notes aren't currently translated. We always want to use the default locale.

This will vary depend on which system note is being used. So far the only one to use to_sentence is the assignee change note. This note is only used in EE, but exists in CE, so the MR is here.

For #63262 (closed).

Screenshots

Before (with user's language set to Spanish):

After:

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry for user-facing changes, or community contribution. Check the link for other scenarios.

Performance and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.