Move Plan specialization triage to 4th and 19th and add information

Updates the backend/frontend specialization label triage issue for Plan to the following:

• Moves dates from 1st and 15th to 4th and 19th so that triage is done during the monthly planning rotation;
• Adds a suggestion to check issues under the Next Up label, so that they appear in the monthly estimation list.

Action items

• (If applicable) Add documentation to the handbook pages for Triage Operations =>
• (If applicable) Identify the affected groups and how to communicate to them:
  • /cc @person_or_group =>
  • Relevant Slack channels =>
  • Week-in-review