build: drop official support for Ubuntu 18.04 and macOS 10.15

Increase the minimum required version of clang to 7.0 on linux, because
older versions fail to compile libstdc++'s implementation of std::variant

Change-Id: I6320973cbe3ff00faa8a36cd0e7ddf564c20d965
diff --git a/.jenkins b/.jenkins
index 0f40e00..92b9dae 100755
--- a/.jenkins
+++ b/.jenkins
@@ -32,6 +32,9 @@
     export DISABLE_HEADERS_CHECK=yes
 fi
 
+# https://reproducible-builds.org/docs/source-date-epoch/
+export SOURCE_DATE_EPOCH=$(git log -1 --format=%ct)
+
 for file in .jenkins.d/*; do
     [[ -f $file && -x $file ]] || continue