GHA/http3-linux: bump to ubuntu-26.04

Before: https://github.com/curl/curl/actions/runs/27772068909
After: https://github.com/curl/curl/actions/runs/27772321661?pr=22089

Closes #22089

.github/workflows/http3-linux.yml

@@ -68,7 +68,7 @@ env:
 jobs:
   build-cache:
     name: 'Build caches'
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-26.04
 
     steps:
       - name: 'cache awslc'
@@ -426,7 +426,7 @@ jobs:
   linux:
     name: ${{ matrix.build.generate && 'CM' || 'AM' }} ${{ matrix.build.name }}
     needs: build-cache
-    runs-on: ubuntu-latest
+    runs-on: ubuntu-26.04
     timeout-minutes: 10
     env:
       CURL_TRACE_PKG_CONFIG: '1'