ci(deps): bump aws-actions/configure-aws-credentials from 5 to 6

v6 moves the action runtime to Node 24, which GitHub-hosted
`ubuntu-latest` runners support natively. The `role-to-assume` and
`aws-region` inputs we pass are unchanged between v5 and v6; the
only other changes in v6 are additive features (transitive tag
keys, corrected role-chaining outputs) that don't affect this
workflow's OIDC → S3 sync path.

Tag: https://github.com/aws-actions/configure-aws-credentials/tree/v6
Changelog: https://github.com/aws-actions/configure-aws-credentials/blob/main/CHANGELOG.md

Author: tony

.github/workflows/docs.yml

@@ -78,7 +78,7 @@ jobs:
 
       - name: Configure AWS Credentials
         if: env.PUBLISH == 'true'
-        uses: aws-actions/configure-aws-credentials@v5
+        uses: aws-actions/configure-aws-credentials@v6
         with:
           role-to-assume: ${{ secrets.LIBTMUX_MCP_DOCS_ROLE_ARN }}
           aws-region: us-east-1