Drop Swift 6.0

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

@@ -11,7 +11,6 @@ jobs:
     name: Unit tests
     uses: ./.github/workflows/unit_tests.yml
     with:
-      linux_6_0_arguments_override: "--explicit-target-dependency-import-check error"
       linux_6_1_arguments_override: "--explicit-target-dependency-import-check error"
       linux_6_2_arguments_override: "--explicit-target-dependency-import-check error"
       linux_6_3_arguments_override: "--explicit-target-dependency-import-check error"

.github/workflows/pull_request.yml

@@ -15,7 +15,6 @@ jobs:
     name: Unit tests
     uses: ./.github/workflows/unit_tests.yml
     with:
-      linux_6_0_arguments_override: "--explicit-target-dependency-import-check error"
       linux_6_1_arguments_override: "--explicit-target-dependency-import-check error"
       linux_6_2_arguments_override: "--explicit-target-dependency-import-check error"
       linux_6_3_arguments_override: "--explicit-target-dependency-import-check error"

Benchmarks/Package.swift

@@ -1,4 +1,4 @@
-// swift-tools-version:6.0
+// swift-tools-version:6.1
 //===----------------------------------------------------------------------===//
 //
 // This source file is part of the SwiftCertificates open source project

Package.swift

@@ -1,4 +1,4 @@
-// swift-tools-version:6.0
+// swift-tools-version:6.1
 //===----------------------------------------------------------------------===//
 //
 // This source file is part of the swift-memcache-gsoc open source project