Remove Redis backwards compatibility in 3 months

[hacdias]

After 2/3 months after the sync service being released (#4), we should remove the backwards compatibility with the old SDK versions from Testground. The required repositories to change are:

• https://github.com/testground/infra: Remove any Redis instance and variables.
• https://github.com/testground/testground: Remove any Redis healthcheck, variables and related.