Update Go version for Docker file carlosedp/golang to 1.23.6

[danielboven]

Background

The Docker image carlosedp/golang is quite extensively used by other Docker images (in the build process). For instance, the shenxn/protonmail-bridge relies on the Go image to build the mail bridge.
For them to be updated, the carlosedp/golang should be bumped to a newer version of Go. I have made small changes to the Dockerfile.golang to allow updating to Go version 1.23.6. I have tested this with success on two different architectures.

Changelog

• Updated VER variable to 1.23.6.
• Changed Golang download URL to current base URL.
• Removed alternative RISCV64 download location (i.e. Go archive in this GitHub repository), as Go releases now ship a RISCV64 version from the downloads by default.
• Changed ENV declarations format in Dockerfile.golang as these were using an outdated format. This prevents the Docker warning LegacyKeyValueFormat.

Small request

If @carlosedp approves this pull request, would you mind updating the Docker image on the container registry (using the command outlined in the top of the Dockerfile.golang)? Thanks in advance!