Update supported Ubuntu releases

[tuzakey]

It appears Ubuntu 20.04 (Focal) is no longer supported by rspamd having gone end of standard support in May 2025. However, the Download page still references Focal and there is no reference to a distro support policy. Focal is still in ESM support from Canonical through 2030 so it would be good to clarify the support policy to avoid confusion.

docs.rspamd.com/docs/downloads.mdx

Lines 78 to 80 in d0fef30

	- **Ubuntu focal** (since 2.5)
	- **Ubuntu jammy** (since 3.3)
	- **Ubuntu noble** (since 3.9)

The dropping of Focal from the apt repo is mentioned here: rspamd/rspamd#5802