docs: remove old environments (#328)

* docs: remove support of galactic

Signed-off-by: rokamu623 <r.okamura.061@ms.saitama-u.ac.jp>

* docs: remove old environment support from faq

Signed-off-by: rokamu623 <r.okamura.061@ms.saitama-u.ac.jp>

* docs: remove old environment support from installation

Signed-off-by: rokamu623 <r.okamura.061@ms.saitama-u.ac.jp>

* docs: change adopting version from 20.04 to 22.04

Signed-off-by: rokamu623 <r.okamura.061@ms.saitama-u.ac.jp>

---------

Signed-off-by: rokamu623 <r.okamura.061@ms.saitama-u.ac.jp>

Author: rokamu623

docs/design/limits_and_constraints/index.md

@@ -7,7 +7,7 @@ CARET only supports environments as follows.
 - Support single host
 - Support FastDDS or CycloneDDS
 - Support only Linux, especially Ubuntu
-- Support Galactic, Humble and Iron
+- Support Humble and Iron
   - Iron support is an experimental feature
 - Require rebuilding of an application with CARET libraries
 

docs/faq/faq.md

@@ -5,7 +5,6 @@
 ### Setup fails
 
 - In case you encounter errors during setup or build process, please make sure to use an appropriate branch for your environment
-  - ROS 2 Galactic, Ubuntu 20.04: `galactic` branch
   - ROS 2 Humble, Ubuntu 22.04: main branch
   - ROS 2 Iron, Ubuntu 22.04: main branch
 - It's also important to delete `./build` `./install` and `./src` directory before rebuilding CARET if you have built CARET using wrong settings

docs/installation/installation.md

@@ -13,7 +13,6 @@ CARET is confirmed to run on the platforms shown in the following table with sup
 | Python3            | 3.10.x            |
 
 The recent version, after v0.3.0, of CARET supports only the combination of ROS 2 Humble and Ubuntu 22.04.  
-If you want to run CARET on ROS 2 Galactic and Ubuntu 20.04, please refer to [documents for v0.2.3](https://tier4.github.io/caret_doc/refs-tags-v0.2.3/tutorials/installation/)  
 We have added an experimental implementation to work with iron.  
 To install for iron, open `for iron` in the Installation section below and follow the instructions.
 
@@ -22,7 +21,7 @@ To install for iron, open `for iron` in the Installation section below and follo
 Installation using meta repository is the least time-consuming way to install CARET.  
 With meta repository and Ansible, you can skip the laborious manual setup which is explained in manual installation(./manual_installation.md) section (written in Japanese).
 
-Please execute the following steps on Ubuntu 20.04. The order is important so that you have to follow the steps in order.
+Please execute the following steps on Ubuntu 22.04. The order is important so that you have to follow the steps in order.
 
 1. Clone `caret` and enter the directory.
 
@@ -31,8 +30,6 @@ Please execute the following steps on Ubuntu 20.04. The order is important so th
    cd ros2_caret_ws
    ```
 
-   `main` branch is dedicated for ROS 2 Humble. If you want to use CARET for ROS 2 Galactic, please execute `git checkout galactic` in `ros2_caret_ws` directory.
-
 2. Create the src directory and clone repositories into it.
 
    CARET uses vcstool to construct workspaces.