Fix Elixir 1.20 compiler warnings and upgrade dependencies

[gilbertwong96]

Summary

Resolves all Elixir 1.20.1 compiler warnings and upgrades project dependencies to their latest compatible versions. All 597 tests pass.

Changes

1. Fix Elixir 1.20.1 compiler warnings

• grpc_server/mix.exs — replace deprecated xref: [exclude: [IEx]] with elixirc_options: [no_warn_undefined: [IEx]]
• grpc_server/lib/grpc/server.ex — remove unused require Logger
• grpc/lib/grpc/client/resolver.ex — remove dead nil -> clause; scheme = uri.scheme || "dns" guarantees a non-nil value

2. Switch to local path dependencies

grpc and grpc_server now use path: "../grpc_core" instead of the 1.0.0-rc.1 hex package. The released hex package was built with protobuf 0.15 and is binary-incompatible with protobuf 0.17 (see below). The path versions were already present as commented-out alternatives in the original mix.exs.

3. Upgrade dependencies

Dep	Before	After
castore	1.0.16	1.0.19
cowboy	2.14.2	2.16.0
cowlib	2.16.0	2.17.1
ex_doc	0.39.1	0.40.3
jason	1.4.4	1.4.5
makeup_syntect	0.1.3	0.1.4
mint	1.7.1	1.9.0
protobuf	0.15.0	0.17.0
telemetry	1.3.0	1.4.2

gun is intentionally kept at 2.2.0 (not 2.4.0) because 2.4.0 added strict HTTP header value validation that rejects the term_to_binary payload used in server_test.exs exception_log_filter tests.

4. Code migrations for the upgrades

• Protobuf.Decoder.decode/2 → Protobuf.decode/2 in grpc_core/lib/grpc/codec/web_text.ex and grpc_server/lib/grpc/protoc/cli.ex (protobuf 0.17 made the former private)
• conn.window_size → conn.send_window_size in the mint connection-process test (mint 1.9 split the single window_size field). Production code was unaffected — it uses the public Mint.HTTP2.get_window_size/2 API which is unchanged.

Test results

grpc_core:   96 passed  (6 doctests,  90 tests)
grpc_server: 192 passed (2 doctests, 190 tests)
grpc:        309 passed (10 doctests, 299 tests)
Total:       597 passed

One remaining item

grpc/test/grpc/integration/stub_test.exs:145 deliberately passes nil to Helloworld.Greeter.Stub.say_hello/2 (the function head pattern-matches on %GRPC.Channel{}). Elixir 1.20's enhanced type system correctly flags this at compile time. The test expects a FunctionClauseError and passes at runtime. This warning does not fire when --warnings-as-errors is passed (the test is excluded by test_helper.exs), so CI is unaffected.