build: migrate from yarn to pnpm, remove CircleCI config

[brandonroberts]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/ngrx/platform/blob/main/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Documentation has been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[x] Build related changes
[x] CI related changes
[ ] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

• yarn 1.x is removed in favor of pnpm for dependency installation and running commands. The docs-app still uses yarn but will be replaced with the new docs site.
• The CircleCI configuration is removed. GitHub Actions were introduced to run our CI/CD.
• Updates to Nx 20.3.0

Closes #

What is the new behavior?

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

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