Update External_gdal.cmake

1 job for release-7.1.1 in 4 minutes and 14 seconds (queued for 40 minutes and 13 seconds)
Status Job ID Name Coverage
  Precheck
failed #31582
fast-build

00:04:14

 
Name Stage Failure
failed
fast-build Precheck
Preparing environment
Running on runner-b7ae13ec-project-311-concurrent-0 via runner-b7ae13ec-ovh-1599645348-375f4f54...
Getting source from Git repository
$ git config --global user.email "otbbot@orfeo-toolbox.org" # collapsed multi-line command
Fetching changes with git depth set to 3...
Initialized empty Git repository in /builds/otb/.git/
Created fresh repository.
fatal: couldn't find remote ref refs/heads/release-7.1.1
ERROR: Job failed: exit code 1