Bug 1843: LTS Support

Revert proxy charm series to bionic as there are some changes that could
adversely affect backwards compatibility

Change-Id: I313f70e3842a31582d469e24174954b55f8ecb6d
Signed-off-by: beierlm <mark.beierl@canonical.com>
diff --git a/n2vc/libjuju.py b/n2vc/libjuju.py
index f570702..6580067 100644
--- a/n2vc/libjuju.py
+++ b/n2vc/libjuju.py
@@ -345,7 +345,7 @@
         db_dict: dict = None,
         progress_timeout: float = None,
         total_timeout: float = None,
-        series: str = "focal",
+        series: str = "bionic",
         wait: bool = True,
     ) -> (Machine, bool):
         """