feat: Switch to OpenSLO SDK

[nieomylnieja]

Summary

• Removed convert subcommand. It was not maintained anymore (for the past 2 years+) and we agreed last year to encourage developing custom converters outside of oslo project.
• Removed pkg. The utlities which were exposed there can now be replaced with OpenSLO SDK.
  If there are functions missing from the SDK, we expect them to be implemented in the SDK, oslo will no longer be a Go library.
• Refactored the whole project, moving pkg to internal, removing bloated packages.
• Added JSON format support to fmt subcommand.
• Copied golangci-lint config from OpenSLO SDK.
• Bumped Go version to 1.23 and removed unused dependencies.

Out of scope

After this PR is merged, but BEFORE a new oslo versions is released we need to cover the bats test cases.
We should also introduce similar dev utilities we currently have in OpenSLO SDK (like devbox).

[nieomylnieja]

Once OpenSLO/OpenSLO#313 is merged, I will create an RC version and use that instead of the commit hash.