Update checkout action to version 3

We were getting a warning with version 2:

The following actions uses node12 which is deprecated and will be forced
to run on node16: actions/checkout@v2. For more info:
https://github.blog/changelog/2023-06-13-github-actions-all-actions-will-run-on-node16-instead-of-node12-by-default/

.github/workflows/db_schema.yml

@@ -25,7 +25,7 @@ jobs:
       PGUSER: consul
       POSTGRES_HOST: postgres
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
       - uses: ruby/setup-ruby@v1
         with:
           bundler-cache: true

.github/workflows/docker.yml

@@ -9,7 +9,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout code
-        uses: actions/checkout@v2
+        uses: actions/checkout@v3
 
       - name: Set up Docker Buildx
         id: buildx

.github/workflows/linters.yml

@@ -6,7 +6,7 @@ jobs:
     runs-on: ubuntu-latest
     steps:
       - name: Checkout code
-        uses: actions/checkout@v2
+        uses: actions/checkout@v3
       - run: |
           git fetch --no-tags --prune origin +refs/heads/*:refs/remotes/origin/*
       - name: Setup Ruby

.github/workflows/tests.yml

@@ -35,7 +35,7 @@ jobs:
         ci_node_total: [5]
         ci_node_index: [0, 1, 2, 3, 4]
     steps:
-      - uses: actions/checkout@v2
+      - uses: actions/checkout@v3
       - uses: ruby/setup-ruby@v1
         with:
           bundler-cache: true