docs: Fixing Ubuntu installation guide

[ScuttoZ]

Important

26.06 FREEZE April 30th: Non-bugfix PRs not ready by this date will wait for 26.09.

RC1 is scheduled on May 14th

The final release is scheduled for June 1st.

Checklist

Before submitting the PR, ensure the following tasks are completed. If an item is not applicable to your PR, please mark it as checked:

• The changelog has been updated in the relevant commit(s) according to the guidelines.
• Tests have been added or modified to reflect the changes.
• Documentation has been reviewed and updated as needed.
• Related issues have been listed and linked, including any that this PR closes.
• Important All PRs must consider how to reverse any persistent changes for tools/lightning-downgrade

Changelog-None

Description

Updating installation guide for Ubuntu. Main highlight: dropped support for versions <22.04.
Tested on docker images.

[daywalker90]

Sorry a few more things:
i verified the steps on ubuntu 22 and 26 and on 22 i stumbled over the issue that i did not install protobuf-compiler because it is on the same line as the rust install which you dont do on 22 because you use rustup. So maybe separate protobuf-compiler to it's own line? Not everyone knows that cargo+rust are both for compiling rust programs and protobuf-compiler is something different.

Also the note for rustup is now duplicate but i like your's more.

After all that please squash this into one commit.

[ScuttoZ]

Sorry a few more things: i verified the steps on ubuntu 22 and 26 and on 22 i stumbled over the issue that i did not install protobuf-compiler because it is on the same line as the rust install which you dont do on 22 because you use rustup. So maybe separate protobuf-compiler to it's own line? Not everyone knows that cargo+rust are both for compiling rust programs and protobuf-compiler is something different.

Does it make sense to place it in the previous block "For development or running tests"?

[daywalker90]

No, because you need it build the rust plugin cln-grpc.

[ScuttoZ]

then I'd add another line after "For ubuntu <25.10 [...]" saying to install protobuf-compiler since it's needed in any case

[daywalker90]

Please no merge commits, only use rebase. Also i just removed the requirement for protobuf-compiler so please rebase and keep it out.

[ScuttoZ]

@daywalker90 Rebased and removed protobuf in b03bed3