Add deprecation warnings (#2248)

Motivation:

v2 has moved to a new repo, grpc-swift-2. 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

Fix decompression of empty messages with a ratio limit (#2246)

Motivation:

The decompressor has a decompression limit to protect against zip bombs.
This can either be absolute or ratio based. It's also possible in gRPC
for a zero length message to be marked as compressed. gRPC attempts to
decompress the zero length message and fails (because zlib wants a
non-zero sized buffer and gRPC won't give it one as the limit is the
buffer size is limited by the `ratio * msg_size` which in this case is
zero).

Modifications:

- If the input to decompress has no length, skip decompression
altogether

Result:

- Can decompress zero length payloads with the ratio limit
- Resolves #2245

Allow padding to be omitted from binary metadata values (#2243)

Motivation:

Binary metadata values are encoded as base64 strings. The gRPC spec
doesn't require that the values are padded. Currently gRPC Swift
requires values to be padded otherwise decoding will fail.

Modifications:

- Allow padding characters to be omitted when decoding base64

Result:

Can decode unpadded binary metadata values

Normalise CI flags (#2240)

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

Generate availability annotations (#2232)

Motivation:

gRPC is moving from specifying the platforms in the package manifest to
annotating code with availability annotations. In order to do this the
generateed code must also be annotated.

Modifications:

- Generate appropriate annotations

Result:

Generated code has availability annotations

---------

Co-authored-by: Gus Cairo <me@gustavocairo.com>

Bump version number to 1.26.0 (#2236)

Motivation:

We plan on tagging a release soon.

Modifications:

- Bump the version to 1.26.0

Result:

The version in the default user-agent string will match the released
version.

Update version to 1.25.0 (#2227)

Convert errors thrown from interceptors (#2209)

Motivation:

gRPC checks whether errors thrown from interceptors are `RPCError` and
otherwise treats them as `unknown` (to avoid leaking internal
information). There is a third possibility: the error is explicitly
marked as being convertible to an `RPCError`. This check is currently
missing when thrown from client/server interceptors.

Modifications:

- Catch `RPCErrorConvertible` in the client/server executors when thrown
from interceptors
- Add tests

Result:

Error information isn't dropped

Fix build issues with static Linux SDK (#2206)

This PR fixes some imports so that grpc-swift can be built using the
static Linux SDK.

Rename nightly_6_1 params to nightly_next (#2203)

Rename nightly_6_1 params to nightly_next; see
apple/swift-nio#3122