chore(enterprise-teams): use field name constants in data_source_github_enterprise_teams

[vmvarela]

Description

Reviewer feedback: use constants for field names (e.g. "name", "slug") in data_source_github_enterprise_teams.go so that renaming a field automatically propagates.

Acceptance Criteria

• Define constants for all field name strings used in d.Set() / schema keys in data_source_github_enterprise_teams.go
• Replace all string literals with the constants
• Verify make build passes

Notes

Upstream PR: integrations#3008