fix(enterprise-teams): remove team_id support — use only team_slug in all resources and data sources

[vmvarela]

Description

The upstream reviewer (@deiga) asked whether we should drop team_id support and accept only team_slug, since the GitHub API is moving away from Team ID APIs.

Decision: use only team_slug for consistency and simplicity.

Acceptance Criteria

• Remove team_id field from data_source_github_enterprise_team
• Remove team_id field from resource_github_enterprise_team
• Remove team_id field from resource_github_enterprise_team_membership
• Remove team_id field from resource_github_enterprise_team_organizations
• Remove findEnterpriseTeamByID helper (no longer needed)
• Update docs accordingly
• Update tests — remove team_id-based test steps

Notes

Upstream PR: integrations#3008