Maybe we should rename 'ExplicitStaleness' to 'IsStaleness'

[lcwangchao]

As the first commit #25206 , ExplicitStaleness is represented for this sql is using stale read explicitly that means it has a 'as of timestamp' clause in it, so it can be false when the txn is a stale read txn.

After this commit #25156 it seems that ExplicitStaleness will be true once it is a in fact stale read. So the name IsStaleness may be betweer.