chore: update dependencies

update Sisimai to 5.2.0

---

updated-dependencies:
- dependency-name: sisimai
  dependency-type: direct:production
  update-type: version-update:semver-patch
...

Signed-off-by: dependabot[bot] <[email protected]>

convert to modular Sinatra app

This attempts to fix an issue with the `request.body` not being a
StringIO instance, but wrapped depending on the application server
in use.

For Webrick, the raw request body seems not to be accessible anymore,
which is a shame. Switching to Puma helps a bit, but getting the raw
body still requires accessing an instance variabale from Rack::Lint...

update dependencies

build(deps): bump sisimai from v5.0.0 to 5.0.1

Bumps [sisimai](https://github.com/sisimai/rb-sisimai) from v5.0.0 to 5.0.1. This release includes the previously tagged commit.
- [Release notes](https://github.com/sisimai/rb-sisimai/releases)
- [Changelog](https://github.com/sisimai/rb-sisimai/blob/5-stable/ChangeLog.md)
- [Commits](sisimai/rb-sisimai@cd166bc...v5.0.1)

---
updated-dependencies:
- dependency-name: sisimai
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <[email protected]>

update to Sisimai 5.0.0

Using sisimai 4.25.16p1

update dependencies

ci: test against MRI 3.2