fix: update minimum gax version to 2.7.0 (#1213)
fix: update minimum gax version to 2.7.0 (#1213)

To fix recent protobuf errors based on https://github.com/googleapis/nodejs-pubsub/pull/1085.

Fixes #1209
googleapis/nodejs-spanner,googleapis/nodejs-spanner,googleapis/nodejs-spanner,googleapis/nodejs-spanner