chore(spanner): regenerate proto files and fix tests

[harshachinta]

This PR does the following,

1. Update proto generated files using latest protoc version.
2. PR chore: prep for switching golint -> go vet #10223 modified the integration test for proto columns that led to test failures spanner: TestIntegration_BasicTypes_ProtoColumns failed #10236 (comment). The modification was done because go vet was throwing some errors as follows

./integration_test.go:2314:57: literal copies lock value from singerProtoMessage: cloud.google.com/go/spanner/testdata/protos.SingerInfo contains google.golang.org/protobuf/internal/impl.MessageState contains sync.Mutex
./integration_test.go:2316:81: literal copies lock value from singerProtoMessage: cloud.google.com/go/spanner/testdata/protos.SingerInfo contains google.golang.org/protobuf/internal/impl.MessageState contains sync.Mutex
./integration_test.go:2336:50: literal copies lock value from singerProtoMessage: cloud.google.com/go/spanner/testdata/protos.SingerInfo contains google.golang.org/protobuf/internal/impl.MessageState contains sync.Mutex
./integration_test.go:2337:54: literal copies lock value from singerProtoMessage: cloud.google.com/go/spanner/testdata/protos.SingerInfo contains google.golang.org/protobuf/internal/impl.MessageState contains sync.Mutex
./integration_test.go:2339:61: literal copies lock value from singerProtoMessage: cloud.google.com/go/spanner/testdata/protos.SingerInfo contains google.golang.org/protobuf/internal/impl.MessageState contains sync.Mutex
./integration_test.go:2342:57: literal copies lock value from singerProtoMessage: cloud.google.com/go/spanner/testdata/protos.SingerInfo contains google.golang.org/protobuf/internal/impl.MessageState contains sync.Mutex

This PR has changes to fix the test failure and avoid go vet issue.

Fixes #10236