README: Drop note about Debian jessie, which is oldoldstable nowadays

As such, hopefully, it's not really used widely anymore. If so, this note isn't really relevant.

assigned to @jvoisin

added 3 commits

• b2f51150...d027008e - 2 commits from branch master
• 1f830bf8 - README: Drop note about Debian jessie, which is oldoldstable nowadays

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enabled an automatic merge when the pipeline for 1f830bf8 succeeds

merged