Remove deprecated usage of pb.Timestamp

[GuessWhoSamFoo]

What steps did you take and what happened:
From github.com/golang/protobuf/ptypes,

// Timestamp converts a timestamppb.Timestamp to a time.Time.
// It returns an error if the argument is invalid.
//
// Unlike most Go functions, if Timestamp returns an error, the first return
// value is not the zero time.Time. Instead, it is the value obtained from the
// time.Unix function when passed the contents of the Timestamp, in the UTC
// locale. This may or may not be a meaningful time; many invalid Timestamps
// do map to valid time.Times.
//
// A nil Timestamp returns an error. The first return value in that case is
// undefined.
//
// Deprecated: Call the ts.AsTime and ts.CheckValid methods instead.

What did you expect to happen:
Remove usage before breaking changes.

Environment:

• Octant version (use octant version): 0.22