gleam: update to version 0.28.1

Changelog:

## v0.28.1 - 2023-04-05

- Fixed a bug where the languag server would unset too many error diagnostics
  when multiple projects are open, more than one have errors, and one of them is
  successfully compiled.
- Fixed a bug where the language server would unset error diagnostics when
  displaying information on hover.

(nikita)