Bump Rakudo Star version

And remove the zef upgrade logic

Author: lizmat

Dockerfile

@@ -1,4 +1,4 @@
-FROM rakudo-star:2023.02
+FROM rakudo-star:2025.06
 WORKDIR /srv
 
 RUN apt-get -y update
@@ -8,7 +8,6 @@ RUN apt-get -y install zstd lrzip libssl-dev build-essential
 RUN git config --global --add safe.directory '*'
 
 COPY META6.json /srv
-RUN zef upgrade zef
 RUN zef install --force --/test HTTP::HPACK # to work around the dependency issue
 RUN zef install --force --/test --deps-only .