grpc-js: Bump version to 1.12.7

[murgatroid99]

To publish some changes

[murgatroid99]

The test jobs are failing because of an @grpc/grpc-js-xds build error, that seems to be related to @types/node versions, but the fix that works locally doesn't seem to work in the CI. I only care about the @grpc/grpc-js build, so I think it's OK to accept that failure.