PEP 751: Make default-group be default-groups

[brettcannon]

Also clarify that using extras and dependency_groups outside of lock file is an error.

• Change is either:
  • To a Draft PEP
  • To an Accepted or Final PEP, with Steering Council approval
  • To fix an editorial issue (markup, typo, link, header, etc)
• PR title prefixed with PEP number (e.g. PEP 123: Summary of changes)

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