feat: change the tag to prevent assignbot assigning a reviewer to 'no-assignbot'

BREAKING CHANGE: assignbot will ignore the 'no-assignbot' tag, and not the 'client-project'

previously the tag was 'client-project', but it was not very well chosen as some
client project may need assignbot's help, and some non-client-project may no
need it.

Let's use a proper tag for this.

closes cubicweb/cubicweb#106
2 jobs for topic/default/change-ignored-tag in 25 seconds (queued for 11 seconds)
Status Job ID Name Coverage
passed lint_black #93875


passed lint_flake8 #93876