Cut 15.0.0 and bump the package version in pyproject.toml, uv.lock, and
CITATION.cff.

Major: one breaking change since 14.0.0. The `approval_ids` biolink override
is removed; the column is no longer a curated edge field, so the
FDA-application-number pass-through no longer bypasses Biolink validation:
an `approval_ids` annotation now emits a relocation warning and folds into
`supporting_text` like any unknown edge field, and strict KGX validation
counts edges carrying it as real `extra_forbidden` failures (#117). Configs
should annotate the real Biolink slot `FDA_regulatory_approvals` instead;
the Annotation validator now canonicalizes any declared casing onto the
allow-list spelling, so the mixed-case slot reaches the final edge verbatim
(#117). Also ships the per-section `category_override` keyed on the resolved
object category, pinning EntityToDiseaseAssociation /
EntityToPhenotypicFeatureAssociation per row so the subclass-only slots
those classes declare survive prune_to_class (#118).

Changelog:
- Versioned the Unreleased section as 15.0.0, gave all three entries their
  PR links, and added a reader/config migration note for the `approval_ids`
  removal (annotate `FDA_regulatory_approvals` with `split_by`, pin the
  subclass via `category_override`).

Docs: none needed here. #117 and #118 already updated
docs/configuration/table.md (the category_override field row and example,
the casing-canonicalization bullet under allowed edge fields, the split_by
mention), docs/cli.md (the pending-fields section no longer names
approval_ids), and docs/agent.md.

Testing:
- uv run pytest -q -> 1088 passed, 15 skipped (94% coverage)
- uv run ruff check . && uv run ruff format --check . && uv run pyright -> clean / 0 errors
- uv lock --check -> up to date
- uv run mkdocs build --strict -> clean