Add deprecation notice to GitHub CLI Authentication (#3538)

Signed-off-by: Timo Sand <timo.sand@f-secure.com>

Author: deiga

docs/index.md

@@ -99,6 +99,8 @@ provider "github" {
 
 ### GitHub CLI Authentication
 
+~> The GitHub CLI authentication fallback is only available when using the legacy client implementation and will be removed in a future major release. To use the legacy client implementation, set `legacy_client = true` in the provider configuration or set the `GITHUB_LEGACY_CLIENT` environment variable to `true`.
+
 When using the GitHub CLI authentication fallback, you can optionally specify the path to the `gh` executable using the `GH_PATH` environment variable. This is useful when the provider cannot properly determine the path to GitHub CLI, such as in cygwin terminals. If not specified, the provider looks for `gh` in your system PATH.
 
 #### .env

templates/index.md.tmpl

@@ -63,6 +63,8 @@ Some API operations may not be available when using a GitHub App installation co
 
 ### GitHub CLI Authentication
 
+~> The GitHub CLI authentication fallback is only available when using the legacy client implementation and will be removed in a future major release. To use the legacy client implementation, set `legacy_client = true` in the provider configuration or set the `GITHUB_LEGACY_CLIENT` environment variable to `true`.
+
 When using the GitHub CLI authentication fallback, you can optionally specify the path to the `gh` executable using the `GH_PATH` environment variable. This is useful when the provider cannot properly determine the path to GitHub CLI, such as in cygwin terminals. If not specified, the provider looks for `gh` in your system PATH.
 
 #### .env