Switch to JDK 11

This mirrors the corresponding change in the main OpenRS2 repository.

It also allows us to switch away from the AdoptOpenJDK builds and use
OpenJDK from Debian instead. This simplifies the process for building
the image.

Dockerfile

@@ -1,12 +1,12 @@
 FROM debian:10-slim
 ENV LANG C.UTF-8
-COPY adoptopenjdk.gpg /etc/apt/trusted.gpg.d/
-RUN apt-get update && \
-  apt-get install -y apt-transport-https ca-certificates && \
-  echo 'deb https://adoptopenjdk.jfrog.io/adoptopenjdk/deb buster main' > /etc/apt/sources.list.d/adoptopenjdk.list && \
+RUN mkdir -p /usr/share/man/man1 && \
   apt-get update && \
-  mkdir -p /usr/share/man/man1 && \
-  apt-get install -y --no-install-recommends adoptopenjdk-8-hotspot && \
-  apt-get install -y --no-install-recommends gcc libc6-dev libgl1-mesa-dev libxt-dev maven && \
-  apt-get autoremove --purge -y && \
+  apt-get install -y --no-install-recommends \
+    gcc \
+    libc6-dev \
+    libgl1-mesa-dev \
+    libxt-dev \
+    maven \
+    openjdk-11-jdk && \
   rm -rf /var/lib/apt/lists/*