Clarify which branches are supported (#1154)

* Update README.md

* Update README.md

Mark branches 1, 2 and 3 as unsupported

README.md

@@ -23,12 +23,12 @@
 
 | Branch | Support Level | Symfony | League Container | psr/log | PHP Versions |
 | ------ | ------------- | ------- | ---------------- | ------------ | ------------ |
-| [4.x](https://github.com/consolidation/robo/tree/4.x) | Stable               | 6 | 3 | 2 - 3 | 8.0 - 8.1 |
-| [3.x](https://github.com/consolidation/robo/tree/3.x) | Important fixes only | 4 - 6 | 3 | 1 - 2 | 7.1 - 8.1 |
-| [2.x](https://github.com/consolidation/robo/tree/2.x) | Not recommended      | 4 - 5 | 2 | 1 - 2 | 7.1 - 7.4 |
-| [1.x](https://github.com/consolidation/robo/tree/1.x) | Not recommended      | 2 - 4 | 2 | 1 - 2 | 5.5 - 7.4 |
+| [4.x](https://github.com/consolidation/robo/tree/4.x) | Stable             | 6 | 3 | 2 - 3 | 8.0 - 8.1 |
+| [3.x](https://github.com/consolidation/robo/tree/3.x) | Not supported      | 4 - 6 | 3 | 1 - 2 | 7.1 - 8.1 |
+| [2.x](https://github.com/consolidation/robo/tree/2.x) | Not supported      | 4 - 5 | 2 | 1 - 2 | 7.1 - 7.4 |
+| [1.x](https://github.com/consolidation/robo/tree/1.x) | Not supported      | 2 - 4 | 2 | 1 - 2 | 5.5 - 7.4 |
 
-All three branches of Robo are currently supported, although the 2.x and 1.x branches receive minimum support. All versions are roughly compatible; the breaking changes introduced at each major version are fairly minor, and typically only affect classes that are not used by most clients.
+All versions are roughly compatible; the breaking changes introduced at each major version are fairly minor, and typically only affect classes that are not used by most clients.
 
 ## Installing