Add the synapse-core team as code owners (#10994)

Co-authored-by: Sean Quah <8349537+squahtx@users.noreply.github.com>

.github/CODEOWNERS

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+# Automatically request reviews from the synapse-core team when a pull request comes in.
+* @matrix-org/synapse-core

changelog.d/10994.misc

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+Add a `CODEOWNERS` file to automatically request reviews from the `@matrix-org/synapse-core` team on new pull requests.