Add process.group fields to generated schemas

[mjwolf]

Add process.group fields as the effective group for the process schema. process.group was (probably mistakenly) being excluded from the schema, as it wasn't part of schemas/subsets/main.yml. The other related process group fields are already included in the schema (real_group, saved_group), and process.group is defined with field re-use in group.yml.

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• Have you signed the contributor license agreement? y
• Have you followed the contributor guidelines? y
• For proposing substantial changes or additions to the schema, have you reviewed the RFC process? n/a
• If submitting code/script changes, have you verified all tests pass locally using make test? y
• If submitting schema/fields updates, have you generated new artifacts by running make and committed those changes? y
• Is your pull request against main? Unless there is a good reason otherwise, we prefer pull requests against main and will backport as needed. y
• Have you added an entry to the CHANGELOG.next.md? y

[trisch-me]

lgtm