Update Slack to Debian Trixie

[tianon]

This is a follow-up to #1069

(see #1069 (comment) in particular)

+ apt-get update
Get:1 http://deb.debian.org/debian trixie InRelease [138 kB]
Get:2 http://deb.debian.org/debian trixie-updates InRelease [47.1 kB]
Get:3 http://deb.debian.org/debian-security trixie-security InRelease [43.4 kB]
Get:4 http://deb.debian.org/debian trixie/main amd64 Packages [9668 kB]
Get:5 http://deb.debian.org/debian trixie-updates/main amd64 Packages [2432 B]
Get:6 http://deb.debian.org/debian-security trixie-security/main amd64 Packages [5304 B]
Get:7 https://packagecloud.io/slacktechnologies/slack/debian jessie InRelease [29.1 kB]
Err:7 https://packagecloud.io/slacktechnologies/slack/debian jessie InRelease
  Sub-process /usr/bin/sqv returned an error code (1), error message is: Signing key on DB085A08CA13B8ACB917E0F6D938EC0D038651BD is not bound:            primary key   because: No binding signature at time 2025-07-24T20:19:54Z   because: Policy rejected non-revocation signature (PositiveCertification) requiring collision resistance   because: SHA1 is not considered secure since 2013-02-01T00:00:00Z
Reading package lists...
W: OpenPGP signature verification failed: https://packagecloud.io/slacktechnologies/slack/debian jessie InRelease: Sub-process /usr/bin/sqv returned an error code (1), error message is: Signing key on DB085A08CA13B8ACB917E0F6D938EC0D038651BD is not bound:            primary key   because: No binding signature at time 2025-07-24T20:19:54Z   because: Policy rejected non-revocation signature (PositiveCertification) requiring collision resistance   because: SHA1 is not considered secure since 2013-02-01T00:00:00Z
E: The repository 'https://packagecloud.io/slacktechnologies/slack/debian jessie InRelease' is not signed.

W: OpenPGP signature verification failed: https://packagecloud.io/slacktechnologies/slack/debian jessie InRelease: Sub-process /usr/bin/sqv returned an error code (1), error message is: Signing key on DB085A08CA13B8ACB917E0F6D938EC0D038651BD is not bound: primary key because: No binding signature at time 2025-07-24T20:19:54Z because: Policy rejected non-revocation signature (PositiveCertification) requiring collision resistance because: SHA1 is not considered secure since 2013-02-01T00:00:00Z