Use the standard definition of errors.Is to implement compareErrors with
≥go1.13. Retain the implementation using golang.org/x/xerrors for versions <go1.13.

This will allow packages using newer Go versions and already relying on
the errors package to get rid of the transitive dependency on
golang.org/x/xerrors.

Add Go 1.16 to the GitHub actions test coverage matrix.

Also switch the gofmt check to only run on Go 1.16, i.e. the latest
supported version.

Specialized diffing strings and slices should occur for
interface types where both values have the same concrete type.
This is especially relevant for protocmp.Transform,
which transforms every proto.Message as a map[string]interface{}.

FormatDiff should only set the verbosity to 3 if the current verbosity
is lower than 3. Otherwise, it may remove an intended higher verbosity
setting causing the reporter output to not differentiate between
two large values that are different at the end.

While we are at it, increase the maxVerbosityPreset to 6.