Disable really old cipher suites

[almarklein]

from https://github.com/almarklein/mypaas/pull/24#issuecomment-778943216 cc @lilahtovmoon

Relevant docs/notes:

• Mozilla distinguishes 3 levels of security/compatibility, it seems most reasonable to go with the recommended intermediate compatibility: https://wiki.mozilla.org/Security/Server_Side_TLS#Intermediate_compatibility_.28recommended.29
• Mozilla even has a template config file for Traefik that we can copy code from: https://ssl-config.mozilla.org/#server=traefik&version=2.4.2&config=intermediate&guideline=5.6
• Traefik docs on TLS: https://doc.traefik.io/traefik/https/tls/
• We already redirect http to https.
• Traefik docs say that With TLS 1.3, the cipher suites are not configurable (all supported cipher suites are safe in this case) -> The specified cipher suites are for 1.2 only.

Also fixes some branch names (we renamed master to main recently), and fix a bug in the scaling deploy for cases in which there are no old containers for that service.

[almarklein]

Before this change:

After: