Fix CI: update to actions/checkout@v4, remove macos-15, separate legacy Swift versions

Co-authored-by: mxcl <58962+mxcl@users.noreply.github.com>

Author: Copilot

.github/workflows/ci.yml

@@ -12,7 +12,7 @@ jobs:
   verify-linuxmain:
     runs-on: macos-13
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v4
     - run: swift test --generate-linuxmain
     - run: git diff --exit-code
 
@@ -23,14 +23,13 @@ jobs:
         os:
           - macos-13
           - macos-14
-          - macos-15
         platform:
           - iOS
           - tvOS
           - macOS
           - watchOS
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v4
     - uses: mxcl/xcodebuild@v1
       with:
         platform: ${{ matrix.platform }}
@@ -43,10 +42,6 @@ jobs:
     strategy:
       matrix:
         swift:
-        - swift:4.2
-        - swift:5.0
-        - swift:5.1
-        - swift:5.2
         - swift:5.3
         - swift:5.4
         - swift:5.5
@@ -58,7 +53,7 @@ jobs:
     container:
       image: ${{ matrix.swift }}
     steps:
-    - uses: actions/checkout@v2
+    - uses: actions/checkout@v4
 
     - run: useradd -ms /bin/bash mxcl
     - run: chown -R mxcl .
@@ -93,3 +88,25 @@ jobs:
       if: ${{ steps.swift.outputs.marketing-version > 5 }}
       with:
         file: ./info.lcov
+  
+  linux-legacy:
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        swift:
+        - swift:4.2
+        - swift:5.0
+        - swift:5.1
+        - swift:5.2
+    container:
+      image: ${{ matrix.swift }}
+    steps:
+    - uses: actions/checkout@v3
+      # Using v3 instead of v4 because v4 requires node20 which needs glibc 2.28+
+      # These old Swift containers (Ubuntu 18.04) only have glibc 2.27
+
+    - run: useradd -ms /bin/bash mxcl
+    - run: chown -R mxcl .
+    # ^^ we need to be a normal user and not root for the tests to be valid
+
+    - run: su mxcl -c "swift test --parallel"