[v7] Replace deb repo with a curl and dpkg -i

[wadells]

v7 backport of #9227

The debian repo is currently broken, and we shouldn't recommend
customers use it until it is fixed.

Contributes to #8166

(cherry picked from commit df31462)

[russjones]

Closing this since since we won't be using this approach. @wadells reopen if needed.

[wadells]

Hi @russjones.

I believe we should take this change for branches 7 and below. I've only re-opened this one, to hash out the discussion though. If we do merge, I'll fix and reopen the others.

Why should we take this? Our users are getting confused about how to install Teleport locked to old versions, because the default linux instructions on old branches suggest using deb https://deb.releases.teleport.dev/ stable main -- which for compatibility reasons will always install the latest teleport. Even in the future when we offer other components like 7 or 7.3, main will track latest. curl + dpkg is a short term docs win that improves the default install instructions for every branch where main installs a later version.

For an example the use of main impacting users, see #9241.

When we have a fix that allows apt to track to an older major/minor version, I'll make sure to update these docs to use apt again. Unfortunately we don't have that today, and with upcoming PTO, I won't have that ready until some time in the new year.

[webvictim]

I agree this approach makes sense in the short term. We've had a lot of issues with people being upgraded to versions of RPMs/DEBs from the repos that they weren't expecting and this seems a low-effort, reasonable solution in the short term.