docs: remove go install instructions from README, note Go version elsewhere (#420)

This may not be the recommended way to install, and the previous getting
started chapter already links to the complete install instructions,
including from prebuilt binaries (as well as the `go install` way).

Author: scop

README.md

@@ -10,21 +10,6 @@ to control your resources from the command line or any compatible interface.
 
 For instructions on how to install `upctl`, configure credentials, and run commands, see [Getting started](https://upcloudltd.github.io/upcloud-cli/) instructions in the documentation.
 
-## Install with go install
-
-Install the latest version of `upctl` with `go install` by running:
-
-```sh
-go install github.com/UpCloudLtd/upcloud-cli/v3/...@latest
-```
-
-Run `upctl version` to verify that the tool was installed successfully and `upctl help` to print usage instructions.
-
-```sh
-upctl version
-upctl help
-```
-
 ## Exit codes
 
 Exit code communicates success or the type and number of failures. Possible exit codes of `upctl` are:

docs/index.md

@@ -74,6 +74,8 @@ using a package manager, Docker image, or from sources with `go install`:
     go install github.com/UpCloudLtd/upcloud-cli/v3/...@latest
     ```
 
+    Make sure to have recent enough Go installed (see `go.mod` in the source tree for the constraints).
+
 === "Docker"
 
     Pull the latest build from GHCR by running: