[rush] Update to pnpm latest version

[TheLarkInn]

Summary

Currently we are running pnpm 6.29.1 inside rushstack. We should update to the latest version (v7.8.0) of .pnpm to take advantage of new features, bugfixes, and performance enhancements. https://github.com/pnpm/pnpm/releases/tag/v7.0.0.

The more people using it, the more we can report back any bugs or peculiarities to @zkochan and friends at pnpm anyways.

Considerations: it looks that Node 12 is not supported in latest pnpm so we can't default it to customers but we can use it on the repo ourselves.

[iclanton]

It may be time to drop official support for Node 12.

[octogonz]

Before using PNPM >=6.32.12, we ideally should first make the Rush PR that sets ignore-compatibility-db=true. (See pnpm/pnpm#5132 for details)

[octogonz]

Here's the PR: #3575