Update GitHub Actions dependencies.

- Configure Dependabot to handle GitHub Actions deps.
- Pin third-party actions to commit SHAs just to be on the safe side
  (like we're supposed to).
- Update to current versions of reusable Actions.
- Explicitly specify to use IPv4 local loopback when running the
  integration testsuite. (It already worked fine before, but prod
  doesn't run v6 so might as well make sure it's the same.)

.github/dependabot.yml

@@ -9,3 +9,7 @@ updates:
     directory: /
     schedule:
       interval: daily
+  - package-ecosystem: github-actions
+    directory: /
+    schedule:
+      interval: weekly

.github/workflows/ci.yml

@@ -15,21 +15,25 @@ jobs:
     name: Test Go
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v3
-      - uses: actions/setup-go@v4
+      - uses: actions/checkout@v4
+        with:
+          show-progress: false
+      - uses: actions/setup-go@v5
         with:
           go-version-file: go.mod
       - run: make unit_tests
       - run: make integration_tests
         env:
-          ROUTER_MONGO_URL: localhost
+          ROUTER_MONGO_URL: 127.0.0.1
   golangci-lint:
     runs-on: ubuntu-latest
     steps:
-      - uses: actions/checkout@v3
-      - uses: actions/setup-go@v4
+      - uses: actions/checkout@v4
+        with:
+          show-progress: false
+      - uses: actions/setup-go@v5
         with:
           go-version-file: go.mod
-      - uses: golangci/golangci-lint-action@v3
+      - uses: golangci/golangci-lint-action@3a919529898de77ec3da873e3063ca4b10e7f5cc  # v3.7.0
         with:
-          version: v1.53
+          version: v1.55.2