Bug 1846: LTS Support

Updates base to Ubuntu 20.04
Updates python to 3.8
Updates pip dependencies to more recent versions
Adds apt cache to stage 2 dockerfile

Fixes bug 1846

Change-Id: I589199979818b0fe5892df1d40849da8f6e06ded
Signed-off-by: beierlm <mark.beierl@canonical.com>
diff --git a/requirements.txt b/requirements.txt
index ac1825e..049c791 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -1,35 +1,3 @@
-certifi==2020.12.5
-    # via requests
-chardet==4.0.0
-    # via requests
-click==7.1.2
-    # via -r requirements.in
-idna==2.10
-    # via requests
-jinja2==2.11.3
-    # via -r requirements.in
-markupsafe==1.1.1
-    # via jinja2
-packaging==20.9
-    # via -r requirements.in
-prettytable==2.1.0
-    # via -r requirements.in
-pycurl==7.43.0.6
-    # via -r requirements.in
-pyparsing==2.4.7
-    # via packaging
-python-magic==0.4.22
-    # via -r requirements.in
-pyyaml==5.4.1
-    # via -r requirements.in
-requests==2.25.1
-    # via -r requirements.in
-urllib3==1.26.4
-    # via requests
-verboselogs==1.7
-    # via -r requirements.in
-wcwidth==0.2.5
-    # via prettytable
 #######################################################################################
 # Copyright ETSI Contributors and Others.
 #
@@ -46,3 +14,35 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 #######################################################################################
+certifi==2021.10.8
+    # via requests
+charset-normalizer==2.0.10
+    # via requests
+click==8.0.3
+    # via -r requirements.in
+idna==3.3
+    # via requests
+jinja2==3.0.3
+    # via -r requirements.in
+markupsafe==2.0.1
+    # via jinja2
+packaging==21.3
+    # via -r requirements.in
+prettytable==3.0.0
+    # via -r requirements.in
+pycurl==7.44.1
+    # via -r requirements.in
+pyparsing==3.0.6
+    # via packaging
+python-magic==0.4.24
+    # via -r requirements.in
+pyyaml==5.4.1
+    # via -r requirements.in
+requests==2.27.1
+    # via -r requirements.in
+urllib3==1.26.8
+    # via requests
+verboselogs==1.7
+    # via -r requirements.in
+wcwidth==0.2.5
+    # via prettytable