Clarify last steps on the monthly template

What does this MR do and why?

Clarifies the last steps on the monthly template

Simplifies the steps associated with adjusting the protected branch rules of the current version.

Examples

Before

• Adjust the protected branch rules for the current version
  • Remove the protected branch rule from the stable branch associated with the previous release (15.11):
    • GitLab protected branch rules
    • Omnibus protected branch rules
  • Add a protected branch rule to allow maintainers to merge into the 16-0-stable-ee branch
    • GitLab protected branch rules
    • Omnibus protected branch rules

After

• Adjust the protected branch rules for the current version
  • Unprotect the 15-11-stable-ee branch
    • GitLab protected branch rules
    • Omnibus protected branch rules
  • Add a protected branch rule to allow maintainers to merge into the 16-0-stable-ee branch
    • GitLab protected branch rules
    • Omnibus protected branch rules

Author Check-list

• [-] Has documentation been updated?