Update repo labels

[kellylee-gds]

Added labels for:

• 'main epic' as we need a way to differentiate between main epics like Design System Day 2023 and the sub-epics like Procure platforms for Design System Day 2023. I can then filter the sprint planning board to only display sub-epics
• 'design system day' for event related activities

[github-actions]

Output from dry-run

---
alphagov/design-system-team-internal
---
Syncing labels for "alphagov/design-system-team-internal"
Validating provided labels
Fetching labels from GitHub
 > Missing: the "main epic" label is missing from the repo. It will be created.
 > Missing: the "design system day" label is missing from the repo. It will be created.

---
alphagov/govuk-design-system
---
Syncing labels for "alphagov/govuk-design-system"
Validating provided labels
Fetching labels from GitHub
 > Missing: the "main epic" label is missing from the repo. It will be created.
 > Missing: the "design system day" label is missing from the repo. It will be created.

---
alphagov/govuk-frontend
---
Syncing labels for "alphagov/govuk-frontend"
Validating provided labels
Fetching labels from GitHub
 > Missing: the "main epic" label is missing from the repo. It will be created.
 > Missing: the "design system day" label is missing from the repo. It will be created.