Updated CI to use v4 instead of v2.

.github/workflows/ci-unittests.yml

@@ -31,12 +31,12 @@ jobs:
     container: python:3.8
     steps:
       # Checks-out your repository under $GITHUB_WORKSPACE, so your job can access it
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
         with:
           fetch-depth: 0
       # Cache local python environment artifacts for the current python version
       # and poetry lockfile hash.
-      - uses: actions/cache@v2
+      - uses: actions/cache@v4
         id: env-cache
         with:
           # Confirmed that the `.local` directory doesn't exist until the
@@ -57,12 +57,12 @@ jobs:
     container: python:3.8
     steps:
       # Checks-out your repository under $GITHUB_WORKSPACE, so your job can access it
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
         with:
           fetch-depth: 0
       # Cache local python environment artifacts for the current python version
       # and poetry lockfile hash.
-      - uses: actions/cache@v2
+      - uses: actions/cache@v4
         id: env-cache
         with:
           # Confirmed that the `.local` directory doesn't exist until the
@@ -96,12 +96,12 @@ jobs:
     # Steps represent a sequence of tasks that will be executed as part of the job
     steps:
       # Checks-out your repository under $GITHUB_WORKSPACE, so your job can access it
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v4
         with:
           fetch-depth: 0
       # Cache local python environment artifacts for the current python version
       # and poetry lockfile hash.
-      - uses: actions/cache@v2
+      - uses: actions/cache@v4
         id: env-cache
         with:
           # Confirmed that the `.local` directory doesn't exist until the