Update gemspec to require pg_query >= v6.1

Bump the minimum required version of pg_query dependency from 5.1.0 to 6.1.0 to resolve compatibility issues with macOS sonoma.

Building the native extension of the pg_query gem is broken on macOS 15.4 and later due to changes introduced in macOS Sonoma.

The underlying issue is fixed in the dependent library, libpg_query,
which addresses strchrnul detection on macOS 15.4:

After upgrading the host to macOS Sequoia 15.4, builds broke due to strchrnul now being available in libc but gated by <string.h> and deployment target constraints.

See also:

• https://github.com/pganalyze/pg_query/issues/327
• https://github.com/pganalyze/libpg_query/issues/276

There are no breaking changes between pg_query v5 and v6, and the most notable change is an update to the libpg_query library, which brings the PostgreSQL v17 parser. AFAIK that should be fine for all our components.