Simplify 'Preparation' steps on Monthly template

What does this MR do?

Simplifies 'Preparation' steps on the Monthly Template.

• Release Managers are on the f_upcoming_release channel, so asking for them to join is a bit redundant.
• Also, it specifies the topic that should be set on the channel

Example

Release 13.2

First steps

• Change #f_upcoming_release topic with /topic 13.2.0: <link_to_this_issue>
• Modify the dates below to accurately reflect the plan of action

The rest of the steps is the same