Revise install instructions (+ install Java 11 everywhere, and update pyenv to use Python 3.8.3) (#3847)

* Use `openjdk-r` ppa for Ubuntu 16.04

* Don't add `deadsnakes` ppa for Ubuntu 20.04

* Use Java 11 for Arch Linux

* Update getting started guide to recommend Ubuntu 20.04 over 18

* Use Java 11 for Fedora Linux

* Use Java 11 for openSUSE Tumbleweed

* remove unneded `-u` for ubuntu 20.04 and 18.04

* use `openjdk-11-jre-headless` in all ubuntus

* use headless jre everywhere

* add non interactive flag to zypper calls

* Add emphasis on info about pyenv's warning

* reverse order of Ubuntu LTS versions

* Install Git 2.11+ with `yum replace` on CentOS/RHEL 7

* Add `--gpg-auto-import-keys` flag to non-interactive `zypper ar` usage

* update pyenv instructions to use Python 3.8.3

* Install git after installing everything else in CentOS 7 instructions

* use zypper flags properly

* improve the sentence a little

* I like consistency

* add missing `sudo`s in CentOS instructions

* add tk-devel to CentOS instructions per pyenv's recommendations

* Install gcc 8 from SCL in CentOS 7

* use git222 instead of git2u on CentOS 7

* Add missing `source ~/.bashrc` line

* use git224 instead of git222 on CentOS 7

* use yum swap over yum-plugin-replace in CentOS 7

docs/getting_started.rst

@@ -270,7 +270,7 @@ If you want to do it, follow these steps.
 2. **Install Linux**
 
   Most of the VPS providers have tools for installing Linux automatically. If
-  you're a beginner, we recommend **Ubuntu 18**.
+  you're a beginner, we recommend **Ubuntu 20.04 LTS**.
 
   For Raspberry Pi users, just install `Raspbian
   <https://www.raspberrypi.org/downloads/raspbian/>`_ on a micro-SD card.