[PATCH rtems-docs] Update package list for Ubuntu

[Ryan Long]

Updated the latest version referenced of Ubuntu to 21.04, updated
package list, and identified what packages need to still be installed
afterwards to pass sb-check.

Closes #4110
---
 user/hosts/posix.rst | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/user/hosts/posix.rst b/user/hosts/posix.rst
index 818eb25..6fbf401 100644
--- a/user/hosts/posix.rst
+++ b/user/hosts/posix.rst
@@ -137,23 +137,23 @@ prefix under your home directory as recommended and end up on the SD card.
 Ubuntu
 ~~~~~~
 
-The latest version is Ubuntu 18.04.1 LTS 64-bit. This section also includes
+The latest version is Ubuntu 21.04.1 LTS 64-bit. This section also includes
 Xubuntu. A minimal installation was used and the following packages installed:
 
 .. code-block:: none
 
-  $ sudo apt-get build-dep build-essential gcc-defaults g++ gdb git \
-  unzip pax bison flex texinfo unzip python3-dev libpython-dev \
-  libncurses5-dev zlib1g-dev
+  $ sudo apt-get build-dep build-essential gcc-defaults g++ gdb unzip \
+  pax bison flex texinfo python3-dev libpython2-dev libncurses5-dev \
+  zlib1g-dev
 
-Note that in previous versions of Ubuntu, the package libpython-dev was
+Note that in previous versions of Ubuntu, the package libpython2-dev was
 python2.7-dev. The name of packages changes over time. You need the
 package with Python development libraries for C/C++ programs. The following
 is needed for recent versions:
 
 .. code-block:: none
 
-  $ sudo apt-get install python-dev
+  $ sudo apt-get install python git
 
 It is likely necessary that you will have to enable the Ubuntu Source
 Repositories.  Users have suggested the following web pages which have
-- 
1.8.3.1