Motivation:

gRPC is moving from specifying the platforms in the package manifest to
annotating code with availability annotations.

Modifications:

- Explicitly annotate public API
- Add '-require-explicit-availability' to CI
- Drop platforms

Result:

- gRPC can be used by packages which don't specify their platforms in
the manifest

Motivation:

`-require-explicit-sendable` is ineffective without `-Xfrontend` but
combining it with `-warnings-as-errors` surfaces a warning from SwiftPM.

Modifications:

- remove `-require-explicit-sendable`, it's not doing anything
- add `-require-explicit-availability` to nightly jobs

Result:

More consistent CI flags