[goal] Migrate testing to ubuntu focal

As per victoria cycle testing runtime and community goal[1]
we need to migrate upstream CI/CD to Ubuntu Focal(20.04).

Fixing:
- bug#1886298
Bump the lower constraints for required deps which added python3.8 support
in their later version.

Story: #2007865
Task: #40228

Change-Id: I697e1d8099a6d776a5b2f9505a99335931e68067

lower-constraints.txt

@@ -10,13 +10,13 @@ eventlet==0.18.2
 extras==1.0.0
 fasteners==0.7.0
 fixtures==3.0.0
-greenlet==0.4.10
+greenlet==0.4.15
 imagesize==0.7.1
 iso8601==0.1.11
 Jinja2==2.10
 keystoneauth1==3.4.0
 linecache2==1.0.0
-MarkupSafe==1.0
+MarkupSafe==1.1.1
 mccabe==0.2.1
 mox3==0.20.0
 msgpack-python==0.4.0
@@ -41,7 +41,7 @@ python-dateutil==2.5.3
 python-mimeparse==1.6.0
 python-subunit==1.0.0
 pytz==2013.6
-PyYAML==3.12
+PyYAML==3.13
 reno==3.1.0
 requests==2.14.2
 requestsexceptions==1.2.0