Phase out old signing key in `securedrop-admin`

[eloquence]

securedrop-admin currently still trusts the old signing key for releases. It's been more than a year since the full key rotation (not to be confused with the recent expiry bump), so I think this can be safely phased out:

https://github.com/freedomofpress/securedrop/blob/2f2db872f60bbc9024e0b35b8ebcdc2f5695acb2/admin/securedrop_admin/init.py#L49-L55

[zenmonkeykstop]

Yup, tis an easy fix/test, adding to 2.5.0