Update SOPS to v3.7.0

[hiddeco]

SOPS v3.7.0 was released about an hour ago, and includes the support for the age encryption format: https://github.com/mozilla/sops/releases/tag/v3.7.0

I tried to do a upgrade using a simple "bump", but due to the modified keyservice package we make use of to be able to decrypt in a contained environment, this does not work without making some modifications to the package first:

internal/sops/keyservice/server.go:68:9: assignment copies lock value to key: go.mozilla.org/sops/v3/keyservice.Key contains google.golang.org/protobuf/internal/impl.MessageState contains sync.Mutex
internal/sops/keyservice/server.go:83:20: call of ks.prompt copies lock value: go.mozilla.org/sops/v3/keyservice.Key contains google.golang.org/protobuf/internal/impl.MessageState contains sync.Mutex
internal/sops/keyservice/server.go:91:22: keyToString passes lock by value: go.mozilla.org/sops/v3/keyservice.Key contains google.golang.org/protobuf/internal/impl.MessageState contains sync.Mutex
internal/sops/keyservice/server.go:100:29: prompt passes lock by value: go.mozilla.org/sops/v3/keyservice.Key contains google.golang.org/protobuf/internal/impl.MessageState contains sync.Mutex
internal/sops/keyservice/server.go:101:27: call of keyToString copies lock value: go.mozilla.org/sops/v3/keyservice.Key contains google.golang.org/protobuf/internal/impl.MessageState contains sync.Mutex
internal/sops/keyservice/server.go:120:9: assignment copies lock value to key: go.mozilla.org/sops/v3/keyservice.Key contains google.golang.org/protobuf/internal/impl.MessageState contains sync.Mutex
internal/sops/keyservice/server.go:135:20: call of ks.prompt copies lock value: go.mozilla.org/sops/v3/keyservice.Key contains google.golang.org/protobuf/internal/impl.MessageState contains sync.Mutex

While determining what must be changed, we should probably also look into if and/or what is required to support the age format as well, assuming that the "missing" items in getsops/sops#688 are not blocking.