v0.21.0

• New! Start a fresh a12nserver just by running npx @curveball/a12n-server
• #412: Fixed a few more PostgreSQL bugs (@AminDhouib)
• #407: Users can be added to groups again with their relative URI (@AminDhouib)
• #399: When trying to add a new OAuth2 client with an existing client_id,
  the server will now emit a 409 instead of a 500 error. (@AminDhouib)
• The requirePkce flag was not respected when creating a new OAuth2 client.