Require Android 5 (SDK level 21); update gradle

Recent okhttp versions require Android 5. Also, a lot of workarounds
(like non-vector graphics for notification icons) which are cumbersome
to maintain have accumulated.

So, DAVx5 will require Android 5 (SDK level 21) from the next release,
which will probably tagged as 3.0.

2.6.x (currently 2.6.5) will be the last branch which supports Android 4.4.
Maybe there will never be a version after 2.6.5, which will still be offered for
Android 4.4 devices. If there's big need for a maintenance release (like severe security
problems), there may be further 2.6.x releases (branched from 2.6.5), but
development and new features will only go to the master branch.

app/build.gradle

@@ -23,7 +23,7 @@ android {
         buildConfigField "long", "buildTime", System.currentTimeMillis() + "L"
         buildConfigField "boolean", "customCerts", "true"
 
-        minSdkVersion 19        // Android 4.4
+        minSdkVersion 21        // Android 5.0
         targetSdkVersion 29     // Android 10.0
         multiDexEnabled true    // >64k methods for Android 4.4
 

cert4android

@@ -1 +1 @@
-Subproject commit b7457528ad6f54db3ec1245a6b80ba4d71ab6327
+Subproject commit 791f78c2aa80baf7f6f786b3780e95b4de201b4e

gradle/wrapper/gradle-wrapper.properties

@@ -3,4 +3,4 @@ distributionBase=GRADLE_USER_HOME
 distributionPath=wrapper/dists
 zipStoreBase=GRADLE_USER_HOME
 zipStorePath=wrapper/dists
-distributionUrl=https\://services.gradle.org/distributions/gradle-5.6.4-all.zip
+distributionUrl=https\://services.gradle.org/distributions/gradle-6.3-all.zip