dns: reduce ttl for fastly hosts

[mweinelt]

This is in preparation of the TLS configuration migration.

[mweinelt]

6 corrections (gandi)
#1: ± MODIFY-TTL artifacts.nixos.org CNAME dualstack.v2.shared.global.fastly.net. ttl=(3600->300)
#2: ± MODIFY-TTL cache.nixos.org CNAME dualstack.v2.shared.global.fastly.net. ttl=(3600->300)
#3: ± MODIFY-TTL cache-staging.nixos.org CNAME dualstack.v2.shared.global.fastly.net. ttl=(3600->300)
#4: ± MODIFY-TTL channels.nixos.org CNAME dualstack.v2.shared.global.fastly.net. ttl=(3600->300)
#5: ± MODIFY-TTL releases.nixos.org CNAME dualstack.v2.shared.global.fastly.net. ttl=(3600->300)
#6: ± MODIFY-TTL tarballs.nixos.org CNAME dualstack.v2.shared.global.fastly.net. ttl=(3600->300)