Update known-bugs.txt

[nielsmh]

Motivation / Problem

Sometimes we forget to update known-bugs.txt when a bug is actually resolved.
Some items in the list are may not even be testable any longer.

Description

• #7235 implemented in version 1.10 actually resolved #5661

Limitations

Possible driver issues that may no longer be relevant but I'm not sure anyone can test for:

• #3294 (I don't remember any issues when running SDL + PulseAudio though)
• #3305 (possibly different for SDL 1 and 2?)
• #3447 (does anyone even attempt to make SDL builds for macOS any longer?)
• #4003 (anyone have a copy of Parallels Desktop on hand?)
• #4511 (I think 32 bpp blitter is default now, so only custom configs would possibly experience this)
• #4587 (anyone use MSYS and can retest this?)
• #4997 (possible SDL 1/2 difference? also recent changes to all video drivers may affect this)

Checklist for review

Some things are not automated, and forgotten often. This list is a reminder for the reviewers.

• The bug fix is important enough to be backported? (label: 'backport requested')
• This PR affects the save game format? (label 'savegame upgrade')
• This PR affects the GS/AI API? (label 'needs review: Script API')
• This PR affects the NewGRF API? (label 'needs review: NewGRF')

[James103]

For #4003, Parallels Desktop has a 14-day free trial available for new users. To Mac developers: Have you already used the free trial or bought Parallels Desktop? If you haven't used the free trial, you may be able to do the following:

1. Download the free trial from the official website.
2. Install and run the downloaded software.
3. Inside Parallels Desktop, download and install Windows 10 (or Linux/Ubuntu or etc).
4. Install OpenTTD inside Parallels Desktop.
5. Run OpenTTD inside Parallels Desktop.

When reporting, note the Parallels Desktop version, the Mac OS version, the Windows 10 version, and the model of the Mac itself (including whether it is x86 or ARM-based) in addition to the OpenTTD version.