[CI]: Bump actions/checkout from 5 to 6

Bumps [actions/checkout](https://github.com/actions/checkout) from 5 to 6.
- [Release notes](https://github.com/actions/checkout/releases)
- [Changelog](https://github.com/actions/checkout/blob/main/CHANGELOG.md)
- [Commits](actions/checkout@v5...v6)

---
updated-dependencies:
- dependency-name: actions/checkout
  dependency-version: '6'
  dependency-type: direct:production
  update-type: version-update:semver-major
...

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

.github/workflows/test.yml

@@ -18,7 +18,7 @@ jobs:
     container:
       image: ghcr.io/openassetio/openassetio-build
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - name: Build
         uses: ./.github/build_openassetio
 
@@ -28,7 +28,7 @@ jobs:
     container:
       image: ghcr.io/openassetio/openassetio-build
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
       - name: Build
         uses: ./.github/build_openassetio_mediacreation
 
@@ -39,7 +39,7 @@ jobs:
       image: aswf/ci-base:2024
     needs: [build-openassetio, build-openassetio-mediacreation]
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
 
       - name: Get OpenAssetIO
         uses: actions/download-artifact@v5.0.0
@@ -72,7 +72,7 @@ jobs:
       image: ghcr.io/openassetio/openassetio-build
     needs: [build-openassetio-mediacreation]
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
 
       # TODO(DF): Remove once fixed upstream. Checkout in a container
       # doesn't use the container's user (appears to use 1001 rather
@@ -116,7 +116,7 @@ jobs:
       image: aswf/ci-base:2024
     needs: [build-openassetio]
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
 
       # TODO(DF): Remove once fixed upstream. Checkout in a container
       # doesn't use the container's user (appears to use 1001 rather
@@ -161,7 +161,7 @@ jobs:
     container:
       image: ghcr.io/openassetio/openassetio-build
     steps:
-      - uses: actions/checkout@v5
+      - uses: actions/checkout@v6
 
       # TODO(DF): Remove once fixed upstream. Checkout in a container
       # doesn't use the container's user (appears to use 1001 rather