qa/distros: change ubuntu_latest to 20.04

Signed-off-by: Sage Weil <sage@newdream.net>

diff --git a/qa/distros/supported/ubuntu_latest.yaml b/qa/distros/supported/ubuntu_latest.yaml
index 64a66d9aa487bdd4542cadd0ea9f16eb5598f36f..75d907e3b9cf89b29f9582831cfc1fa1ada69533 120000 (symlink)
--- a/qa/distros/supported/ubuntu_latest.yaml
+++ b/qa/distros/supported/ubuntu_latest.yaml
@@ -1 +1 @@
-../all/ubuntu_18.04.yaml
\ No newline at end of file
+../all/ubuntu_20.04.yaml
\ No newline at end of file