Add wiki page diff url to Slack notification message

What does this MR do?

In this MR, we're adding the specific wiki page diff url to the notification message sent to Slack.

Screenshots (strongly suggested)

Before, the Slack notification was:

The new message is:

Where Custom changes points to the wiki page diff for the last version.

Does this MR meet the acceptance criteria?

Conformity

• I have included a changelog entry, or it's not needed. (Does this MR need a changelog?)
• I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.

Related to #20811 (closed)