Renovate updates of go mod's toolchain directive lead to incorrect go versions used in the pipelines

Our pipelines run with with a set of Go versions to ensure Gitaly builds with the supported versions. This is done by installing the correct version of Go and running the pipeline with it.

Renovate bot however is creating MRs that update the toolchain directive in go.mod to later version. This causes our pipelines to run with wrong versions, for example a Go 1.23 pipeline was installing and executing with Go 1.24 here.

We should fix this to ensure our pipelines run with correct versions. There are two ways we can go about this:

• Remove the toolchain directives from the Go mod files, and stop Renovate from introducing them. This way the Go version our pipelines install is the one that is used.
• Override the toolchain directive in the go.mod file through an environment variable to match the correct version for the pipeline.