github: update checkout action to v4 to silence warnings

Problem: Github warns that

 Node.js 16 actions are deprecated. Please update the following
 actions to use Node.js 20: actions/checkout@v3.

Update to checkout@v4 to silence the warnings.

Author: grondo

.github/workflows/asan.yml

@@ -6,7 +6,7 @@ jobs:
     runs-on: ubuntu-latest
     timeout-minutes: 60
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
       with:
         ref: ${{ github.event.pull_request.head.sha }}
         fetch-depth: 0

.github/workflows/codeql.yml

@@ -38,7 +38,7 @@ jobs:
 
     steps:
     - name: Checkout repository
-      uses: actions/checkout@v3
+      uses: actions/checkout@v4
 
     # Initializes the CodeQL tools for scanning.
     - name: Initialize CodeQL

.github/workflows/main.yml

@@ -9,7 +9,7 @@ jobs:
     runs-on: ubuntu-latest
     if: github.event_name == 'pull_request'
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
       with:
         ref: ${{ github.event.pull_request.head.sha }}
         fetch-depth: 0
@@ -19,7 +19,7 @@ jobs:
   spelling:
     runs-on: ubuntu-20.04
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
     - name: Check Spelling
       uses: crate-ci/typos@e477391cc0243daaeeb154a7bfa67cb7d6fc5831 # v1.16.8
 
@@ -30,7 +30,7 @@ jobs:
     - uses: actions/setup-python@v4
       with:
         python-version: 3.6
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
       with:
         ref: ${{ github.event.pull_request.head.sha }}
         fetch-depth: 0
@@ -44,7 +44,7 @@ jobs:
     name: flux-sched check
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
       with:
         ref: ${{ github.event.pull_request.head.sha }}
         fetch-depth: 0
@@ -62,7 +62,7 @@ jobs:
     name: flux-accounting check
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
       with:
         ref: ${{ github.event.pull_request.head.sha }}
         fetch-depth: 0
@@ -78,7 +78,7 @@ jobs:
     name: flux-pmix check
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
       with:
         ref: ${{ github.event.pull_request.head.sha }}
         fetch-depth: 0
@@ -100,7 +100,7 @@ jobs:
     name: flux-pam check
     runs-on: ubuntu-latest
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
       with:
         ref: ${{ github.event.pull_request.head.sha }}
         fetch-depth: 0
@@ -122,7 +122,7 @@ jobs:
     outputs:
       matrix: ${{ steps.set-matrix.outputs.matrix }}
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
       with:
         ref: ${{ github.event.pull_request.head.sha }}
         fetch-depth: 0
@@ -149,7 +149,7 @@ jobs:
       fail-fast: false
     name: ${{matrix.name}}
     steps:
-    - uses: actions/checkout@v3
+    - uses: actions/checkout@v4
       with:
         ref: ${{ github.event.pull_request.head.sha }}
         fetch-depth: 0
@@ -160,7 +160,7 @@ jobs:
         github.ref != 'refs/heads/master'
       run: |
         # Ensure git-describe works on a tag.
-        #  (checkout@v3 action may have left current tag as
+        #  (checkout@v4 action may have left current tag as
         #   lightweight instead of annotated. See
         #   https://github.com/actions/checkout/issues/290)
         #