In IPv6-only environment, can't connect to Redis due to DNS only looking for A records

This issue has been migrated from [#10694](https://github.com/matrix-org/synapse/issues/10694).

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### Description

My redis instance has only an IPv6 address. The matrix-synapse container is throwing up a DNS lookup error:
`
2021-08-24 13:30:55,585 - twisted - 258 - ERROR - sentinel- Message: 'Connection to redis server %s failed: %s'
2021-08-24 13:30:55,585 - twisted - 258 - ERROR - sentinel- Arguments: ("b'matrix-test-redis-master':6379", DNSLookupError(b'matrix-test-redis-master'))
`
It also doesn't work with a FQDN IPv6-only address.

I did a tcpdump of DNS traffic in the container, it's only searching for A records (not AAAA).

- **Platform**:

It's running in a IPv6-only Kubernetes cluster.