ci: Drop Android API 21 build jobs

API version 21 was used for Android 5.0
(https://developer.android.com/studio/releases/platforms#5.0), which is
no longer supported
(https://en.wikipedia.org/wiki/Android_version_history).

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>

.gitlab-ci.yml

@@ -265,21 +265,6 @@ valgrind:
     paths:
       - "_build/meson-logs"
 
-cross-android_api21_arm64:
-  extends:
-    - .cross-build-linux
-    # The API 28 build is run on each commit, so the API 21 build can be run
-    # less regularly to save CI resources. The chance of a regression on API 21
-    # and not API 28 is tiny.
-    - .only-schedules
-  image: $ANDROID_IMAGE
-  script:
-    # FIXME: add --werror
-    # We use -Diconv=auto to test that we successfully detect that iconv is not
-    # provided by android api 21, and detect the external iconv instead.
-    - meson ${MESON_COMMON_OPTIONS} --cross-file=/opt/cross_file_android_arm64_21.txt -Diconv=auto _build
-    - ninja -C _build
-
 cross-android_api28_arm64:
   extends:
     - .cross-build-linux

.gitlab-ci/android-ndk.Dockerfile

@@ -60,7 +60,6 @@ ENV ANDROID_NDK_PATH /opt/android-ndk
 COPY android-download-ndk.sh .
 RUN ./android-download-ndk.sh
 COPY android-setup-env.sh .
-RUN ./android-setup-env.sh arm64 21
 RUN ./android-setup-env.sh arm64 28
 RUN rm -rf $ANDROID_NDK_PATH
 

.gitlab-ci/android-setup-env.sh

@@ -38,21 +38,6 @@ export LD=$target_host-ld
 export STRIP=$target_host-strip
 export PATH=$PATH:$toolchain_path/bin
 
-# Cross build libiconv when using API level <= 28.
-# Newer Android has it in its libc already.
-if [ "$api" -lt "28" ]; then
-  wget --quiet http://ftp.gnu.org/pub/gnu/libiconv/libiconv-1.15.tar.gz
-  echo "1233fe3ca09341b53354fd4bfe342a7589181145a1232c9919583a8c9979636855839049f3406f253a9d9829908816bb71fd6d34dd544ba290d6f04251376b1a  libiconv-1.15.tar.gz" | sha512sum -c
-  tar xzf libiconv-1.15.tar.gz
-  pushd libiconv-1.15
-  ./configure --host="${target_host}" --prefix="${prefix_path}"  --libdir="${prefix_path}/lib64"
-  make
-  make install
-  popd
-  rm libiconv-1.15.tar.gz
-  rm -r libiconv-1.15
-fi
-
 # Cross build libffi
 wget --quiet https://github.com/libffi/libffi/releases/download/v3.3-rc0/libffi-3.3-rc0.tar.gz
 echo "e6e695d32cd6eb7d65983f32986fccdfc786a593d2ea18af30ce741f58cfa1eb264b1a8d09df5084cb916001aea15187b005c2149a0620a44397a4453b6137d4  libffi-3.3-rc0.tar.gz" | sha512sum -c