fix: bump Go module path from v6 to v7

[Copilot]

The goreleaser publish job was failing because the module path (/v6) didn't match the release tag (v7.0.2). Go enforces that the major version suffix in the module path must equal the tag's major version.

Changes

• go.mod: updated module declaration from github.com/sil-org/personnel-sync/v6 → v7
• 19 .go files: updated all internal import paths accordingly

failed to proxy module: go: github.com/sil-org/personnel-sync/v6@v7.0.2:
invalid version: module path includes a major version suffix,
so major version must match