Pin NPM version installed by CI pipelines

Nobody has claimed this yet.

When LoopBack's CI pipelines started incorporating GitHub Actions, `npm install --global npm` was added as a step performed before installation. This was to prevent an issue where, under certain circumstances, the Mocha tests would silently pass without even executing the tests themselves. However, a side-effect was that this would install the latest NPM version. This meant that CI pipelines which still tested against Node.js v10 broke due to incompatibility with the newly-relesaed NPM v8.

The NPM version policy is as follows:

1. Use the latest supported NPM version for that specific Node.js version.
2. Where there may be a gap between NPM major versions in the test matrix, add an additional x86 Linux test matrix permutation on GitHub Actions.

This issue is to track the updating of CI pipelines to use pinned NPM major versions.

Repos to update:
- [x] strong-soap (PR: https://github.com/loopbackio/strong-soap/pull/343)
- [x] loopback-connector (PR: https://github.com/loopbackio/loopback-connector/pull/192)
- [x] loopback-connector-mongodb
- - [x] v6.x (PR: https://github.com/loopbackio/loopback-connector-mongodb/pull/652)
- - ~v5.x~ (Not applicable)
- [x] loopback-connector-grpc (PR: https://github.com/loopbackio/loopback-connector-grpc/pull/17)

Review the GitHub Actions CI pipeline definitions in the listed repositories, starting with the npm install --global npm step and each Node.js test-matrix entry. Compare them with the stated NPM version policy and confirm the listed repositories are updated; the checklist and linked PRs indicate this work is already complete.

Written by the indexing model from the issue text.