use apksigner to sign index-v2 with modern, supported algorithms

The current signing method uses apksigner to sign the JAR so that it
will automatically select algorithms that are compatible with Android
SDK 23, which added the most recent algorithms:
https://developer.android.com/reference/java/security/Signature

This signing method uses then inherits the default signing algothim
settings, since Java and Android both maintain those.  That helps
avoid a repeat of being stuck on an old signing algorithm.  That means
specifically that this call to apksigner does not specify any of the
algorithms.

The old indexes must be signed by SHA1withRSA otherwise they will no
longer be compatible with old Androids.

apksigner 30.0.0+ is available in Debian/bullseye, Debian/buster-backports,
Ubuntu 21.10, and Ubuntu 20.04 from the fdroid PPA.  Here's a quick way to
test:

for f in `ls -1 /opt/android-sdk/build-tools/*/apksigner | sort ` /usr/bin/apksigner; do printf "$f : "; $f sign --v4-signing-enabled false; done

closes #1005

.gitlab-ci.yml

@@ -125,7 +125,7 @@ ubuntu_lts_ppa:
     - ./run-tests
 
 
-# Test using Ubuntu/bionic LTS (supported til 2022) with all depends
+# Test using Ubuntu/bionic LTS (supported til April, 2023) with all depends
 # from pypi.  The venv is used to isolate the dist tarball generation
 # environment from the clean install environment.
 ubuntu_bionic_pip:
@@ -145,6 +145,11 @@ ubuntu_bionic_pip:
     - tar tzf dist/fdroidserver-*.tar.gz
     # back to bare machine to act as user's install machine
     - $pip install --upgrade pip setuptools wheel  # make this go away: "error: invalid command 'bdist_wheel'"
+
+    - export ANDROID_HOME=/opt/android-sdk
+    - $pip install sdkmanager
+    - sdkmanager 'build-tools;30.0.0'
+
     - $pip install dist/fdroidserver-*.tar.gz
     - tar xzf dist/fdroidserver-*.tar.gz
     - cd fdroidserver-*