Upgrade Badger storage to v4

[yurishkuro]

Renovate bot created this PR #5610, but it's not really doing anything since we'd also need to change the imports:

$ grx badger/v3 plugin/storage/badger
plugin/storage/badger/spanstore/rw_internal_test.go:23:	"github.com/dgraph-io/badger/v3"
plugin/storage/badger/spanstore/cache_test.go:20:	"github.com/dgraph-io/badger/v3"
plugin/storage/badger/spanstore/cache.go:22:	"github.com/dgraph-io/badger/v3"
plugin/storage/badger/spanstore/writer.go:24:	"github.com/dgraph-io/badger/v3"
plugin/storage/badger/spanstore/reader.go:27:	"github.com/dgraph-io/badger/v3"
plugin/storage/badger/factory.go:27:	"github.com/dgraph-io/badger/v3"
plugin/storage/badger/samplingstore/storage_test.go:22:	"github.com/dgraph-io/badger/v3"
plugin/storage/badger/samplingstore/storage.go:23:	"github.com/dgraph-io/badger/v3"

They may be compatibility issues both in the APIs and with the data. In particular, we will need to verify that data created with the current version can still be used with v4.

[hellspawn679]

i would like to work on this issue