CI: No more Ubuntu 20.04 [skip ci]

Ubuntu 20.04's default Python doesn't deal well with the type annotations
used in btest/Files/wstest.py. Given that it's about to be EOL, just remove it.

.cirrus.yml

@@ -357,14 +357,6 @@ ubuntu22_spicy_head_task:
   skip: $CIRRUS_BRANCH =~ 'release/.*'
   << : *BENCHMARK_ONLY_IF_TEMPLATE
 
-ubuntu20_task:
-  container:
-    # Ubuntu 20.04 EOL: April 2025
-    dockerfile: ci/ubuntu-20.04/Dockerfile
-    << : *RESOURCES_TEMPLATE
-  << : *CI_TEMPLATE
-  << : *SKIP_TASK_ON_PR
-
 alpine_task:
   container:
     # Alpine releases typically happen every 6 months w/ support for 2 years.

ci/ubuntu-20.04/Dockerfile

@@ -1,40 +0,0 @@
-FROM ubuntu:20.04
-
-ENV DEBIAN_FRONTEND="noninteractive" TZ="America/Los_Angeles"
-
-# A version field to invalidate Cirrus's build cache when needed, as suggested in
-# https://github.com/cirruslabs/cirrus-ci-docs/issues/544#issuecomment-566066822
-ENV DOCKERFILE_VERSION 20241024
-
-RUN apt-get update && apt-get -y install \
-    bc \
-    bison \
-    bsdmainutils \
-    ccache \
-    cmake \
-    curl \
-    flex \
-    g++ \
-    gcc \
-    git \
-    jq \
-    lcov \
-    libkrb5-dev \
-    libmaxminddb-dev \
-    libpcap-dev \
-    libssl-dev \
-    libzmq3-dev \
-    make \
-    python3.9 \
-    python3.9-dev \
-    python3-pip\
-    ruby \
-    sqlite3 \
-    swig \
-    unzip \
-    wget \
-    zlib1g-dev \
-  && apt autoclean \
-  && rm -rf /var/lib/apt/lists/*
-
-RUN pip3 install websockets junit2html