Add deprecation warnings (#50)

Motivation:

gRPC Swift v2 has moved to a new repo, grpc-swift-2 and as a result this
package has a new major version. That's not all that discoverable.

Modifications:

Deprecate commonly used high-level types with a link to a forums post
explaining the move.

Result:

Users are notified about the move

Move to grpc-swift-2 (#49)

Motivation:

To support incremental migration, v2 has moved to the 'grpc-swift-2'
package.

Modifications:

- Update dependencies
- Update generated code
- Re-baseline availability

Result:

Easier to migrate from gRPC Swift 1 to 2

Move availability inline (#47)

Motivation:

It's hard for packages to incrementally adopt gRPC with platforms in the
package manifest as it requires packages to include platforms in their
manifest or mint a new major version.

Modifications:

- Move platform availability onto source code
- Check annotations in CI

Result:

Easier to adopt

Relax dependency requirements (#36)

Bump dependency versions (#33)

Depend on beta 3 tag (#28)

Bump versions (#22)

Bump dependency version (#15)

Set grpc-swift version to 2.0.0-alpha.1 (#5)

Motivation:

We just tagged grpc-swift 2.0.0-alpha.1, we should use it.

Modifications:

Update version requirement.

Result:

Better version management.