Bump ubuntu from 24.04 to 26.04

Bumps ubuntu from 24.04 to 26.04.

---
updated-dependencies:
- dependency-name: ubuntu
  dependency-version: '26.04'
  dependency-type: direct:production
...

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

Author: dependabot[bot]

Dockerfile

@@ -6,7 +6,7 @@
 FROM ghcr.io/omec-project/bess_build:260501@sha256:bf262866cddbd01d0e5011a189066c72b1a5474d146cc6f42ff6464b9ea6d019 AS bess-build
 
 # Stage bess: creates the runtime image of BESS
-FROM ubuntu:24.04@sha256:d1e2e92c075e5ca139d51a140fff46f84315c0fdce203eab2807c7e495eff4f9 AS bess
+FROM ubuntu:26.04@sha256:f3d28607ddd78734bb7f71f117f3c6706c666b8b76cbff7c9ff6e5718d46ff64 AS bess
 
 ENV DEBIAN_FRONTEND=noninteractive