Drop Swift 6.0 (#61)

Motivation:

Swift 6.0 is no longer supported, we should bump the tools version and
remove it from our CI.

Modifications:

* Bump the Swift tools version to Swift 6.1
* Remove Swift 6.0 jobs where appropriate in main.yml, pull_request.yml

Result:

Code reflects our support window.

Author: rnro

.github/workflows/main.yml

@@ -14,7 +14,6 @@ jobs:
     name: Unit tests
     uses: apple/swift-nio/.github/workflows/unit_tests.yml@main
     with:
-      linux_6_0_arguments_override: "-Xswiftc -warnings-as-errors --explicit-target-dependency-import-check error -Xswiftc -require-explicit-sendable"
       linux_6_1_arguments_override: "-Xswiftc -warnings-as-errors --explicit-target-dependency-import-check error -Xswiftc -require-explicit-sendable"
       linux_6_2_arguments_override: "-Xswiftc -warnings-as-errors --explicit-target-dependency-import-check error -Xswiftc -require-explicit-sendable"
       linux_6_3_arguments_override: "-Xswiftc -warnings-as-errors --explicit-target-dependency-import-check error -Xswiftc -require-explicit-sendable"

.github/workflows/pull_request.yml

@@ -18,7 +18,6 @@ jobs:
     name: Unit tests
     uses: apple/swift-nio/.github/workflows/unit_tests.yml@main
     with:
-      linux_6_0_arguments_override: "-Xswiftc -warnings-as-errors --explicit-target-dependency-import-check error -Xswiftc -require-explicit-sendable"
       linux_6_1_arguments_override: "-Xswiftc -warnings-as-errors --explicit-target-dependency-import-check error -Xswiftc -require-explicit-sendable"
       linux_6_2_arguments_override: "-Xswiftc -warnings-as-errors --explicit-target-dependency-import-check error -Xswiftc -require-explicit-sendable"
       linux_6_3_arguments_override: "-Xswiftc -warnings-as-errors --explicit-target-dependency-import-check error -Xswiftc -require-explicit-sendable"