ci: require gcovr 5.1 for code coverage reports

Stop passing `-j`, it wasn't very useful and it's broken in gcovr 5.1

Change-Id: I17ffb97c2dc9cd8db7c900fbbd4ab5f8f92184dd

.jenkins.d/00-deps.sh

@@ -36,7 +36,7 @@ elif has Ubuntu $NODE_LABELS; then
     case $JOB_NAME in
         *code-coverage)
             sudo apt-get -qy install lcov python3-pip
-            pip3 install --user --upgrade --upgrade-strategy=eager 'gcovr~=5.0'
+            pip3 install --user --upgrade --upgrade-strategy=eager 'gcovr~=5.1'
             ;;
         *Docs)
             sudo apt-get -qy install doxygen graphviz python3-pip

.jenkins.d/30-coverage.sh

@@ -4,15 +4,14 @@ set -ex
 if [[ $JOB_NAME == *"code-coverage" ]]; then
     # Generate an XML report (Cobertura format) and a detailed HTML report using gcovr
     # Note: trailing slashes are important in the paths below. Do not remove them!
-    gcovr -j$WAF_JOBS \
-          --object-directory build \
+    gcovr --object-directory build \
           --exclude tests/ \
           --exclude websocketpp/ \
           --exclude-throw-branches \
           --exclude-unreachable-branches \
-          --print-summary \
+          --cobertura build/coverage.xml \
           --html-details build/gcovr/ \
-          --xml build/coverage.xml
+          --print-summary
 
     # Generate a detailed HTML report using lcov
     lcov --quiet \