Fix CI - bump MSRV to 1.87

[thejpster]

• Bumps MSRV to 1.87 to avoid failure seen in Support directory deletion #210
• Removes core-error feature, because MSRV is high enough we can just always turn it on

[jannic]

Wouldn't it be more robust to set resolver.incompatible-rust-versions="fallback" (or resolver = "3" or edition = '2024' which changed the default to fallback)?

That way, cargo will automatically select versions of dependencies that are compatible with the current compiler.
Bumping the rustc version used in CI only helps with the new MSRV of heapless, but will break again once any dependency updates to a more recent MSRV.

[jannic]

While setting resolver="3" may still be a good idea, there's nothing wrong with this PR. Approved.