revise supported platforms

[morgo]

Change Request

Please answer the following questions before submitting your issue. Thanks!

1. Describe what you find is inappropriate or missing in the existing docs.

On https://docs.pingcap.com/tidb/dev/hardware-and-software-requirements it says that Ubuntu 16.04 or later is required.

2. Describe your suggestion or addition.

For the "dev" and "5.0" branches of TiDB, they will be released after Ubuntu 16.04 reaches the end of its regular life (april 2021) and goes into security fixes only mode. So I am suggesting that the version of Ubuntu can be bumped to 18.04 for these future releases.

The existing TiDB releases can stay as-is, since it's kind of weird to upgrade it mid-cycle.

3. Provide some reference materials (documents, websites, etc) if you could.

https://ubuntu.com/about/release-cycle

[morgo]

/assign @alex-quan-001

[alex-quan-001]

We should follow the lifecycle of the operating system to update our test environment, the latest version is no longer supported within 6 months or within 2 months the official version is no longer supported (depending on the lifecycle of the operating system, some up to 10 years, some may be shorter).

cc @shuke987 I am suggesting that the version of Ubuntu can be bumped to 18.04 for these future releases too.

[dveeden]

In addition I would mention "CentOS 8 Stream". This is because CenOS 8 Linux support ends this year and users should probably pick CentOS 8 Stream instead. Note that RHEL/OEL support should remain the same. See also https://centos.org/distro-faq/