Revise manual cadence deprecation message

What does this MR do and why?

• Revise the manual cadence deprecation messages to clarify a conversion to automatic is not required to continue use manual cadence.

Screenshots or screen recordings

Before	After

How to set up and validate locally

Make sure the group you are using has a manual iterations cadence. If one doesn't exist, create one via Rails console.

% rails c

Execute the following code:

group_id = 72 # Use the id of your group
Iterations::Cadence.new(group: Group.find(group_id), start_date: Date.new(2022, 5, 1), iterations_in_advance: 2, automatic: false, title: "Manual cadence").save!(validate: false)

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.