Promote process.io and process.tty fields to GA

[mjwolf]

Remove process.io and process.tty fields from beta, and promote to GA.

These fields have stabilized and are used in released features, so they can leave beta.

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

[github-actions[bot]]

Documentation changes preview: https://ecs_2317.docs-preview.app.elstc.co/diff

[trisch-me]

@norrietaylor it still says it is beta in current release (8.12)

[norrietaylor]

Good catch @trisch-me, it appears the docs were not updated when we removed the beta tags from in-product.

@nick-alayil is going to work with @benironside to fix this inconsistency.