hardware-and-software-requirements: Update supported platforms (#5587)

hardware-and-software-requirements.md

@@ -12,16 +12,21 @@ As an open source distributed NewSQL database with high performance, TiDB can be
 
 | Linux OS Platform        | Version      |
 | :-----------------------:| :----------: |
-| Red Hat Enterprise Linux | 7.3 or later |
-| CentOS                   | 7.3 or later |
-| Oracle Enterprise Linux  | 7.3 or later |
+| Red Hat Enterprise Linux | 7.3 or later 7.x releases |
+| CentOS                   | 7.3 or later 7.x releases |
+| Oracle Enterprise Linux  | 7.3 or later 7.x releases |
 | Ubuntu LTS               | 16.04 or later |
 
 > **Note:**
 >
 > - For Oracle Enterprise Linux, TiDB supports the Red Hat Compatible Kernel (RHCK) and does not support the Unbreakable Enterprise Kernel provided by Oracle Enterprise Linux.
 > - A large number of TiDB tests have been run on the CentOS 7.3 system, and in our community there are a lot of best practices in which TiDB is deployed on the Linux operating system. Therefore, it is recommended to deploy TiDB on CentOS 7.3 or later.
 > - The support for the Linux operating systems above includes the deployment and operation in physical servers as well as in major virtualized environments like VMware, KVM and XEN.
+> - Red Hat Enterprise Linux 8.0, CentOS 8 Stream, and Oracle Enterprise Linux 8.0 are not supported yet as the testing of these platforms is in progress.
+> - Support for CentOS 8 Linux is not planned because its upstream support ends on December 31, 2021.
+> - Support for Ubuntu 16.04 will be removed in future versions of TiDB. Upgrading to Ubuntu 18.04 or later is strongly recommended.
+
+Other Linux OS versions such as Debian Linux and Fedora Linux might work but are not officially supported.
 
 ## Software recommendations