feat: Removed ben and added k9ert keys for releasing

[k9ert]

The installation and update process is done with two different binaries. Both binaries contain pubkeys which makes it possible to check signatures on the device when upgrading.
This PR is removing Ben's key and it's adding k9ert's key. It gets checked with each update. That means that this PR gets relevant for new installations with a new release and for upgrades one release after.

This release is ugrading the specter-bootloader from fc6e61 to 6e727 which contains 2 PRs:

• #20 fix tests on linux
• #22 update signing keys, add xpubconvert tool for convenience

Most of the files come from the test PR in the test directory. The relevant change is in the pubkeys.c. The xpubconvert.py tool can be used to convert an xpub to the format needed in the pubkeys.c.

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[3rdIteration]

Ack

[k9ert]

• signature test would be very reasonable to have for all participants
• Also doing an initial firmware and test an upgrade.
• Figure out how to build a firmware which is upgrading the bootloader

[k9ert]

Figure out how to build a firmware which is upgrading the bootloader

see #323 and this comment proves it.

The rest of the two requirments will get fulfilled while executing on #330.

[tadeubas]

Is that correct? I see this PR is updating the specter-bootloader dependency to commit from August cryptoadvance/specter-bootloader@6e727e0 but the last commit is from November cryptoadvance/specter-bootloader@9f9d8c7

[miketlk]

LGTM. Thank you, @k9ert and sorry for the delay with reviewing.