Release policy: Mark 2.1 and 3.1 as EOL

As much as I'd like to keep supporting them, platforms such as Google Play or
Apple Store move too fast for our limited resources, and backporting relevant
changes to make those years old releases still viable proves to be unrealistic.

I'm also no longer able to make 2.1.x builds easily as that buildsystem relied
on Travis CI which discontinued its support for open source.

With three releases supported in parallel (4.0, 3.4 and 3.3), we have enough
work on our hands.

(cherry picked from commit 50f809abbb1497275b397091b8855bceb8e98baf)

about/release_policy.rst

@@ -77,7 +77,7 @@ who need maintenance updates.
 Whenever a new major version is released, we make the previous stable branch a
 long-term supported release, and do our best to provide fixes for issues
 encountered by users of that branch who cannot port complex projects to the new
-major version. This is the case for the 2.1 branch, and will be the case for
+major version. This was the case for the 2.1 branch, and will be the case for
 the latest 3.x stable branch by the time Godot 4.0 is released.
 
 In a given minor release series, only the latest patch release receives support.
@@ -99,13 +99,11 @@ on GitHub.
 | Godot 3.2   | January 2020         | |eol| No longer supported as fully superseded by the compatible 3.3      |
 |             |                      | release (last update: 3.2.3).                                            |
 +-------------+----------------------+--------------------------------------------------------------------------+
-| Godot 3.1   | March 2019           | |partial| Only critical, security and platform support fixes             |
-|             |                      | (last update: 3.1.2).                                                    |
+| Godot 3.1   | March 2019           | |eol| No longer supported (last update: 3.1.2).                          |
 +-------------+----------------------+--------------------------------------------------------------------------+
 | Godot 3.0   | January 2018         | |eol| No longer supported (last update: 3.0.6).                          |
 +-------------+----------------------+--------------------------------------------------------------------------+
-| Godot 2.1   | July 2016            | |partial| Only critical, security and platform support fixes             |
-|             |                      | (last update: 2.1.6).                                                    |
+| Godot 2.1   | July 2016            | |eol| No longer supported (last update: 2.1.6).                          |
 +-------------+----------------------+--------------------------------------------------------------------------+
 | Godot 2.0   | February 2016        | |eol| No longer supported (last update: 2.0.4.1).                        |
 +-------------+----------------------+--------------------------------------------------------------------------+