Test on go1.21

[pgporada]

Updates CI tests to also test against go1.21.

[pgporada]

The test failures don't appear to be related to this change seeing as it's also failing on go1.19 and go1.20.

 •••••redis: 2023/08/18 16:37:17 pubsub.go:168: redis: discarding bad PubSub connection: EOF
redis: 2023/08/18 16:37:17 pubsub.go:168: redis: discarding bad PubSub connection: EOF
redis: 2023/08/18 16:37:17 pubsub.go:168: redis: discarding bad PubSub connection: EOF
redis: 2023/08/18 16:37:17 pubsub.go:168: redis: discarding bad PubSub connection: EOF

[peczenyj]

I think this was already done in another Pull Request

[pgporada]

Work was duplicated in #2697, but that's alright because the upgrade happened.