docs: reconcile changelog into a real 1.0.0 and note target frameworks

No 1.0.0 was ever published (latest on NuGet is 0.3.0) and the [1.0.0] entry
was aspirational/stale. Consolidate everything since 0.3.0 into a single dated
1.0.0 entry: target frameworks + packaging, the extensibility/filters/DX work
from this cycle, and the folded-in modernization. Note net8.0/net10.0 support
in the README.

feat(search): add Bitbucket Server code search API client, models, an…

…d tests

- Add request/response models for POST /rest/search/latest/search
- Add SearchCodeAsync and IsSearchAvailableAsync to BitbucketClient
- Register search models in BitbucketJsonContext for source-generated JSON
- Add WireMock fixtures plus integration + serialization coverage for the undocumented API

Refactor and enhance code quality with new features (#2)

ready for release 0.2.0

ci: publish to GitHub Packages alongside NuGet.org