chore(maintenance): add core team to code owners

[dreamorosi]

Description of your changes

This PR adds the @aws-powertools/core GitHub team to the CODEOWNERS file so that if one of the members of that team reviews a PR, the requirement of having at least one approver is fulfilled.

This is not the case today, and this essentially equates to requiring multiple reviews which is not what we want at this stage.

Related issues, RFCs

Issue number: closes #2098

Checklist

• My changes meet the tenets criteria
• I have performed a self-review of my own code
• I have commented my code where necessary, particularly in areas that should be flagged with a TODO, or hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my change is effective and works
• The PR title follows the conventional commit semantics

Breaking change checklist

Is it a breaking change?: NO

• I have documented the migration process
• I have added, implemented necessary warnings (if it can live side by side)

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[sonarqubecloud]

Quality Gate passed

Issues
0 New issues

Measures
0 Security Hotspots
No data about Coverage
No data about Duplication

See analysis details on SonarCloud

[dreamorosi]

I already talked with Governance offline and @sthulb is ok with the proposed change. Merging.