ci: bump actions/{checkout,upload-artifact} to v6

This solves "Node.js 20 actions are deprecated." message.

Signed-off-by: Marcin Niestroj <marcin.niestroj@canonical.com>

Author: mniestroj

.github/workflows/bsim.yml

@@ -48,7 +48,7 @@ jobs:
     runs-on: ubuntu-24.04
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
         with:
           path: pouch-gateway
 

.github/workflows/build.yml

@@ -62,7 +62,7 @@ jobs:
     runs-on: ubuntu-24.04
     steps:
       - name: Checkout repository
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
         with:
           path: pouch-gateway
 
@@ -114,7 +114,7 @@ jobs:
           echo "ARTIFACT_TARGET=${TARGET//\//-}${SUFFIX}" >> $GITHUB_OUTPUT
 
       - name: Upload Artifacts
-        uses: actions/upload-artifact@v4
+        uses: actions/upload-artifact@v6
         with:
           name: ${{ matrix.app }}-${{ steps.build.outputs.ARTIFACT_TARGET }}
           path: |

.github/workflows/coverity.yml

@@ -12,7 +12,7 @@ jobs:
     container: golioth/golioth-coverity-base:89df175
 
     steps:
-      - uses: actions/checkout@v4
+      - uses: actions/checkout@v6
         with:
           path: pouch-gateway
       - name: Init and update west
@@ -46,7 +46,7 @@ jobs:
             --form version="0" \
             --form description="Description" \
             https://scan.coverity.com/builds?project=golioth%2Fpouch-gateway
-      - uses: actions/upload-artifact@v4
+      - uses: actions/upload-artifact@v6
         with:
           name: coverity_output
           path: gateway.tgz

.github/workflows/lint.yml

@@ -12,7 +12,7 @@ jobs:
 
     steps:
       - name: Checkout Repository
-        uses: actions/checkout@v4
+        uses: actions/checkout@v6
         with:
           # Fetch depth must be greater than the number of commits included in the push in order to
           # compare against commit prior to merge. 15 is chosen as a reasonable default for the