MT#62936 Bump REFRESHED_AT to force docker image rebuild

It's necessary to build docker image with 'MT#62936 Wait for memtest
starts' commit.

Change-Id: If9f50e8a241542d29a91d2e62d5fd02e2aa568c8

t/Dockerfile

@@ -5,7 +5,7 @@ FROM docker.mgm.sipwise.com/sipwise-trixie:latest
 # is updated with the current date. It will force refresh of all
 # of the base images and things like `apt-get update` won't be using
 # old cached versions when the Dockerfile is built.
-ENV REFRESHED_AT=2025-06-13
+ENV REFRESHED_AT=2025-07-21
 
 RUN apt-get update && apt-get install --assume-yes git make gcc dpkg-dev