Update Linux system requirements for WebKit2

[rmartin16]

Describe the bug

The story for ensuring system requirements on Linux for WebKit2 are installed has been complicated by the release of 4.1.

Many of the distros are offering 4.0 and 4.1 while others (notably Ubuntu 24.04) only offer 4.1; see beeware/beeware#333 (comment).

Steps to reproduce

See documentation.

Expected behavior

The documentation needs to be updated so users know how to install WebKit2.

I think as much as possible, we should push people towards 4.1.

Another complicating factor will likely be the introduction of GTK4...since I believe that's version 6.x of GTK's WebKit2. It hasn't been clear to me if the plan is to only support GTK4...or if a transition period will support both.

Screenshots

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Environment

• Operating System: ubuntu 24.04
• Toga: 0.4.2

Logs

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Additional context

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