chore: tell renovate to run go mod tidy after updating

[ctreatma]

We periodically have to manually run go mod tidy to clean up our dependencies because Renovate doesn't run that by default for its update PRs. This runs one last manual tidy and updates the Renovate configuration to tell Renovate to clean up after itself from now on.

Reference: https://docs.renovatebot.com/configuration-options/#postupdateoptions

[displague]

I've noticed this problem and fix-up pattern before. Glad to see it addressed.