[fenix] Bump taskgraph to latest revision (https://github.com/mozilla-mobile/fenix/pull/8137)

* Bump taskgraph to latest revision

* Change Flank url

The repository moved from github.com/TestArmada/flank to
github.com/Flank/flank, and the curl command in the ui-tests Dockerfile
currently fails because of the redirection.

.taskcluster.yml

@@ -8,7 +8,7 @@ tasks:
     - $let:
           taskgraph:
               branch: taskgraph
-              revision: bb532a33ad32213f3a396346b26abd28fb87a975
+              revision: 7a51e874018f13aa236f5dc64ae7bbcaf942fcf3
           trustDomain: mobile
       in:
           $let:

taskcluster/docker/ui-tests/Dockerfile

@@ -24,7 +24,7 @@ RUN curl https://dl.google.com/dl/cloudsdk/channels/rapid/downloads/google-cloud
     && ${TEST_TOOLS}/google-cloud-sdk/install.sh --quiet \
     && ${TEST_TOOLS}/google-cloud-sdk/bin/gcloud --quiet components update
 
-RUN URL_FLANK_BIN=$(curl -s "https://api.github.com/repos/TestArmada/flank/releases" | grep "browser_download_url*" | grep "${FLANK_VERSION}" | sed -r "s/\"//g" | cut -d ":" -f3) \
+RUN URL_FLANK_BIN=$(curl -s "https://api.github.com/repos/Flank/flank/releases" | grep "browser_download_url*" | grep "${FLANK_VERSION}" | sed -r "s/\"//g" | cut -d ":" -f3) \
     && wget "https:${URL_FLANK_BIN}" -O ${TEST_TOOLS}/flank.jar \
     && chmod +x ${TEST_TOOLS}/flank.jar