Add migration note for Ubuntu 14.04

This commit adds a migration note to the docs for Ubuntu 14.04 seeing as
it will not be supported after April 30, 2019 by upstream.

Author: jasontedor

buildSrc/src/main/resources/minimumGradleVersion

@@ -1 +1 @@
-5.2.1
+5.3

docs/reference/migration/migrate_7_0/packaging.asciidoc

@@ -16,3 +16,9 @@ in `/etc/systemd/system/elasticsearch.service.d/override.conf`.
 
 The tar package previously included files in the `bin` directory meant only
 for windows. These files have been removed. Use the `zip` package instead.
+
+[float]
+==== Ubuntu 14.04 is no longer supported
+
+Ubuntu 14.04 will reach end-of-life on April 30, 2019. As such, we are no longer
+supporting Ubuntu 14.04.