Remove support for Guile 1.8.

Hello!

This removes support for Guile 1.8, which has long been deprecated and unmaintained (last release in Dec. 2010).

This change doesn't necessarily have to go in 3.7.7; it could wait until 3.8 in case someone somewhere relies on Guile 1.8 support in 3.7.

Thoughts?

Ludo'.

Checklist

• Commits have Signed-off-by: with name/author being identical to the commit author
• Code modified for feature
• Test suite updated with functionality tests
• Test suite updated with negative tests
• Documentation updated / NEWS entry present (for non-trivial changes)
• CI timeout is 2h or higher (see Settings/CICD/General pipelines/Timeout)

Reviewer's checklist:

• Any issues marked for closing are addressed
• There is a test suite reasonably covering new functionality or modifications
• Function naming, parameters, return values, types, etc., are consistent and according to CONTRIBUTION.md
• This feature/change has adequate documentation added
• No obvious mistakes in the code