requirements: move Ubuntu-20.04 into unsupported category due to python

From: Paul Gortmaker <paul.gortmaker@kernel.org>

Bitbake commit 0d2e682d00df ("bitbake: lib/bb: Bump minimum python
version requirement to 3.9" -- poky commit 67566c7410e1) means that
Ubuntu 20.04 is no longer supported out-of-the-box as detailed further
in the respective commit log.

While it is true that it can still be used with buildtools, that can
also be said for nearly every other unsupported distro out there,
including Ubuntu 18.04 that we listed as unsupported some years ago.
Hence continuing to list it as supported with the new python requirement
in place does not make sense.

So we move it alongside of the 18.04 entry, into the "possibly tested,
but you get what you get" category.  And add an explicit mention of the
buildtools, so people have a thread to pull on.

Signed-off-by: Paul Gortmaker <paulg@kernel.org>

Hi,

On Fri Mar 21, 2025 at 8:27 PM CET, Paul Gortmaker via lists.yoctoproject.org wrote:
> From: Paul Gortmaker <paul.gortmaker@kernel.org>
>
> Bitbake commit 0d2e682d00df ("bitbake: lib/bb: Bump minimum python
> version requirement to 3.9" -- poky commit 67566c7410e1) means that
> Ubuntu 20.04 is no longer supported out-of-the-box as detailed further
> in the respective commit log.
>
> While it is true that it can still be used with buildtools, that can
> also be said for nearly every other unsupported distro out there,
> including Ubuntu 18.04 that we listed as unsupported some years ago.
> Hence continuing to list it as supported with the new python requirement
> in place does not make sense.
>
> So we move it alongside of the 18.04 entry, into the "possibly tested,
> but you get what you get" category.  And add an explicit mention of the
> buildtools, so people have a thread to pull on.
>
> Signed-off-by: Paul Gortmaker <paulg@kernel.org>
>
> diff --git a/documentation/ref-manual/system-requirements.rst b/documentation/ref-manual/system-requirements.rst
> index 93b95a7fb251..3d0d26e2f08b 100644
> --- a/documentation/ref-manual/system-requirements.rst
> +++ b/documentation/ref-manual/system-requirements.rst
> @@ -58,8 +58,6 @@ Supported Linux Distributions
>  Currently, the &DISTRO; release ("&DISTRO_NAME;") of the Yocto Project is
>  supported on the following distributions:
>  
> --  Ubuntu 20.04 (LTS)
> -
>  -  Ubuntu 22.04 (LTS)
>  
>  -  Ubuntu 24.04 (LTS)
> @@ -93,11 +91,15 @@ organizations publishing them no longer make updates publicly available:
>  
>  -  Ubuntu 18.04 (LTS)
>  
> +-  Ubuntu 20.04 (LTS)
> +
>  -  Ubuntu 23.04
>  
>  Note that the Yocto Project doesn't have access to private updates
>  that some of these versions may have. Therefore, our testing has
> -limited value if you have access to such updates.
> +limited value if you have access to such updates.  Also, buildtools

s/buildtools/:term:`buildtools`/, so users are hinted towards what this is.

I can make that change before merging if that's ok to you.

Antonin