Update all documentation to trust our new signing key

git grep -i -E 'D9C1|tails-signing|signing key' currently says:

• wiki/src/doc/get/trusting_tails_signing_key.mdwn
• wiki/src/doc/get/verify_the_iso_image_using_the_command_line.html: note that the expected output is more complex due to signatures done with a subkey, and there are actually two different possible expected outputs
• wiki/src/doc/get/verify_the_iso_image_using_other_operating_systems.html
• wiki/src/doc/about/openpgp_keys.mdwn
• wiki/src/contribute/release_process.mdwn
• wiki/src/contribute/release_process/test.mdwn
• wiki/src/contribute/git.mdwn

… but better look again when implementing it :)

Feature Branch: feature/8740-new-signing-key-phase-2

Parent Task: #8740 (closed)

Original created by @intrigeri on 8734 (Redmine)