Update all test certificates and add maintainence documentation

[complexspaces]

This PR does several things, with the goal of both unbreaking CI today and making future maintenance easier. These include:

• Reducing the number of chains we need to maintain in the test data
• Simplifying tests with less constants
• Adding documentation for the test certificate update process
• Switching away from 1Password-related certificate chains to ones for AWS themselves. I did this for two reasons:
  • Trying to special-case 1Password less in the repository since its now under the rustls org instead.
  • Earlier this year I learned that we (1Password) have not yet enabled TLS 1.3 support for our production servers, so the certificate update script wasn't able to communicate with my.1password.com because it only supports TLS 1.3 by default. I could have just enabled the tls12 feature but it seemed cleaner to just part with it and use a web server that has its TLS configurations updated more often.
  • Updating the real world and mock certificates, as both have since expired this summer.

Commit-by-commit review is recommended.

[djc]

LGTM, thanks for getting this done!

[complexspaces]

Thanks for the fast approval! Please bear with me as I get the various platform-specific compilation failures fixed 😅.

[complexspaces]

@djc Do you mind double checking my approach in ade5bb0? I'm not super happy with it but we haven't yet worked out #179 and we need to do something to get CI passing again at minimum.

[djc]

It seems reasonable-ish at least!