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 branch/default in 27 seconds (queued for 19 seconds)
Status Job ID Name Coverage
  Lint
passed #96565
lint_black

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passed #96566
lint_flake8

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