Move review ownership to CODEOWNERS

[Jesper Schulz-Wedde (JesperSchulz)]

Summary

• add @microsoft/dynamics-365-business-central as the repository fallback when no more specific CODEOWNER exists
• move AL review routes from the Official Branches ruleset into CODEOWNERS, including API, disabled tests, upgrade, GDL, demo-tool/data, and dotnet*.al ownership
• keep existing Power BI, AI/developer-experience, e-document, modules, entitlement, SCM, Integrations, Finance, and specialist mappings authoritative through CODEOWNERS' last-match-wins ordering
• make ruleset-controlled special paths explicitly ownerless in CODEOWNERS so they do not request both the fallback team and the ruleset reviewer
• remove redundant Power BI rules and condense the guidance

Ruleset coordination

Update the Official Branches ruleset (ID 17834252) as follows:

• enable require_code_owner_review
• remove the blanket * required reviewer
• remove the AL file/folder-specific required reviewers after migrating their ownership to CODEOWNERS
• retain only these special-path required reviewers:
  • App Rulesets: /src/rulesets/
  • Code Review Tools: /tools/Code Review
  • Engineering Systems: .AL-Go/, *.ps1, /.azuredevops, /.github, /build, /CODEOWNERS

Unlike the ruleset's additive * required-reviewer entry, the CODEOWNERS fallback is replaced by any later matching rule. The 539 tracked files covered by the retained special-path rules are explicitly ownerless in CODEOWNERS; every other tracked file resolves to a CODEOWNER.

Fixes AB#647781.