Remove v2 code from v1

Motivation:

There's a bug in SwiftPM which prohibits the Xcode build system from
being used if a dependency explicitly declares which Swift language mode
is being used, regardless of its value. A number of users are running
into this problem, and while it's not a gRPC Swift bug we can work
around the issue by removing the v2 code from the v1 branch and removing
the Swift 6 specific manifest.

Modifications:

- Remove all v2 code
- Remove workarounds in the SwiftPM build plugin which avoid deprecation
  warnings. It turns out that the guard was incorrect anyway: the API
  isn't guarded by the compiler version, but the package tools version.

Result:

Possible to build v1 with SwiftPM using the Xcode build system.

Author: glbrntt

.github/workflows/ci.yaml

@@ -124,16 +124,12 @@ jobs:
         include:
           - image: swiftlang/swift:nightly-jammy
             swift-tools-version: '6.0'
-            supports-v2: true
           - image: swift:6.0-jammy
             swift-tools-version: '6.0'
-            supports-v2: true
           - image: swift:5.10.1-noble
             swift-tools-version: '5.10'
-            supports-v2: false
           - image: swift:5.9-jammy
             swift-tools-version: '5.9'
-            supports-v2: false
     name: Integration Tests on ${{ matrix.image }}
     runs-on: ubuntu-latest
     container:
@@ -144,9 +140,6 @@ jobs:
       run: apt update && apt install -y protobuf-compiler
     - name: SwiftPM plugin test (v1)
       run: ./scripts/run-plugin-tests.sh ${{ matrix.swift-tools-version }} "v1"
-    - name: SwiftPM plugin test (v2)
-      if: ${{ matrix.supports-v2 }}
-      run: ./scripts/run-plugin-tests.sh ${{ matrix.swift-tools-version }} "v2"
     - name: Build without NIOSSL
       run: swift build
       env: