Upgrade Cypress GitHub Action

Description

[x] Upgrades workflow to use the cypress-io/github-action@v6 GitHub Action.
[x] Shares install job between internal and external jobs
[x] Triggers workflow on PR creation and status update (e.g. from draft to ready for review)
[x] Runs Percy only on internal workflow when PR is ready for review
[x] Adds more containers to run cypress jobs to reduce execution time from 20 min to 10 min

Manual functionality testing
- [x] I have tested these changes manually
- [ ] Creator of the original issue (or service owner) has been contacted for manual testing (or will be contacted when released in alpha)
- [ ] No testing done/necessary
Automated tests
- [ ] Unit test(s) have been added/updated
- [ ] Cypress E2E test(s) have been added/updated
- [x] No automatic tests are needed here (no functional changes/additions)
- [ ] I want someone to help me make some tests
UU/WCAG (follow these guidelines until we have our own)
- [ ] I have tested with a screen reader/keyboard navigation/automated wcag validator
- [x] No testing done/necessary (no DOM/visual changes)
- [ ] I want someone to help me perform accessibility testing
User documentation @ altinn-studio-docs
- [ ] Has been added/updated
- [x] No functionality has been changed/added, so no documentation is needed
- [ ] I will do that later/have created an issue
Support in Altinn Studio
- [x] Issue(s) created for support in Studio
- [ ] This change/feature does not require any changes to Altinn Studio
Sprint board
- [x] The original issue (or this PR itself) has been added to the Team Apps project and to the current sprint board
- [ ] I don't have permissions to do that, please help me out
Labels
- [x] I have added a kind/* label to this PR for proper release notes grouping
- [ ] I don't have permissions to add labels, please help me out