Fix ecosystem support table (#17829)

Private repo support:
Elixir - no
yarn - yes

Author: hubwriter

data/reusables/dependabot/supported-package-managers.md

@@ -10,7 +10,7 @@ Bundler | `bundler` | v1 | | **✓** |
 Cargo | `cargo` | v1 | **✓** | |
 Composer | `composer` | v1, v2  | **✓** | |
 Docker | `docker` | v1 | **✓** | |
-Elixir | `mix` | v1 | **✓** | |
+Elixir | `mix` | v1 | | |
 Elm | `elm` | v0.18, v0.19 | **✓** | |
 git submodule | `gitsubmodule` | N/A (no version) | **✓** | |
 GitHub Actions | `github-actions` |  N/A (no version) | **✓** | |
@@ -24,7 +24,7 @@ pipenv | `pip` | <= 2018.11.26 | | |
 pip-compile | `pip` | 5.5.0 | | |
 poetry | `pip` | v1 | | |
 Terraform | `terraform` | <= 0.11 | **✓** | |
-yarn | `npm` | v1 | | |
+yarn | `npm` | v1 | **✓** | |
 
 (A) {% data variables.product.prodname_dependabot %} doesn't run Gradle but supports updates to the following files: `build.gradle` and `build.gradle.kts` (for Kotlin projects).