ci: Drop --enable-c++20 option

[hebasto]

This option has ceased to exist since #28349.

[DrahtBot]

The following sections might be updated with supplementary metadata relevant to reviewers and maintainers.

Code Coverage

For detailed information about the code coverage, see the test coverage report.

Reviews

See the guideline for information on the review process.

Type	Reviewers
ACK	maflcko

If your review is incorrectly listed, please react with 👎 to this comment and the bot will ignore it on the next update.

Conflicts

Reviewers, this pull request conflicts with the following ones:

• #28710 (Remove the legacy wallet and BDB dependency by achow101)

If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

[maflcko]

I kept it, so that it can be turned into an c++23 option once there is cmake, but it is fine to remove the no-op in the meantime.

ACK 64722e4

[fanquake]

This will all have to be rewritten in any case, so fine to dump it.