doc: Update Distributions page with 24.04

[chrisschnabel]

The distributions page references 18.04 as a distribution that implies free support within the 5 year window, and does not include 24.04

Ubuntu 18.04.6 LTS, Ubuntu 20.04.6 LTS, and Ubuntu 22.04.3 LTS are the LTS versions of Ubuntu and receive updates for five years.

While 18.04 is still on the microsoft marketplace, I'm unclear whether it's supported or not. 24.04 definitely is. To make this page more evergreen, another possibility may be to refer to all distributions published under Canonical in the Microsoft Store.

I believe it is also relevant information to point to the Ubuntu without a version number, that seems to always track the latest LTS.

Also, I don't really understand what this sentence means. Unless it's just me, perhaps it requires some word smithing:

Upgrades to future LTS releases will not be proposed.

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Document: reference/distributions.md