ci: bump actions/checkout from v2 to v6

actions/checkout v2 runs on Node 12, which is end-of-life in GitHub
Actions runners and emits a deprecation warning on every job. v6 is
the current major release and runs on the actively supported Node
runtime. The action is invoked with no inputs here, so no migration
work beyond the version bump is needed.

msys2/setup-msys2 is left at v2: that is still its current major
version.

Assisted-by: Claude:claude-opus-4-7

Author: Youw

.github/workflows/linux.yml

@@ -15,7 +15,7 @@ jobs:
     steps:
       # Checks-out your repository under $GITHUB_WORKSPACE, so your job
       # can access it
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v6
 
       - name: setup prerequisites
         shell: bash

.github/workflows/macos.yml

@@ -15,7 +15,7 @@ jobs:
     steps:
       # Checks-out your repository under $GITHUB_WORKSPACE, so your job
       # can access it
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v6
 
       - name: setup prerequisites
         shell: bash

.github/workflows/msys2.yml

@@ -8,7 +8,7 @@ jobs:
       run:
         shell: msys2 {0}
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v6
       - uses: msys2/setup-msys2@v2
         with:
           msystem: MINGW64