Deprecate the ptypes package since all the equivalent functionality
is now directly generated with the well-known types themselves.

Use protodesc.ToFileDescriptorProto to retrieve the raw descriptors
for legacy support instead of the undocumented ProtoLegacyRawDesc method
that we expect v2 to provide.

This change will cause the legacy proto package to incur a dependency
on the descriptorpb package.

Travis-CI is dead. GitHub actions is the new hotness.
The minimally supported version is Go1.11 to make use of modules.

…haler (#1300)

When a JSON null is encountered for a field which implements JSONPBUnmarshaler,
jsonpb will now call the unmarshal method, instead of just skipping the field.

Use the new deprecation feature to mark this module as deprecated.
See https://golang.org/issue/40357.

Considerations:
* google.golang.org/protobuf/cmd/[email protected] and below
used to generate a hard dependency on github.com/golang/protobuf,
which would be frustrating since it would force an explicit dependency
on a deprecated module. However, that is no longer the case in v1.26.0.
* google.golang.org/protobuf and github.com/golang/protobuf have
a cyclic dependency on each other. However, this should not be a problem
since proposal 40357 only marks direct dependencies in the go.mod file,
rather than all transitive dependencies in the go.sum file.