fix: releases replication key

[TylerHillery]

I have updated the replication_key for GitHub releases to created_at field vs the published_at field.

The pubished_at field can be null according to the REST API endpoints for releases

This was causing the below error

{"TypeError: '>' not supported between instances of 'NoneType' and 'str'"}

created_at field can't be null so this should fix that.

[sonarcloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

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

See analysis details on SonarCloud

[edgarrmondragon]

Makes sense to me. Thanks @TylerHillery!