Wiki instructions for updating `npm`'s bundled copy of `node-gyp` need updating

See this comment: https://github.com/nodejs/node-gyp/issues/2354#issuecomment-810429130

To restate the main changes needed:

• The Windows instructions are incorrect, as they use the -g flag (meaning, to modify a standalone, global install of node-gyp), whereas the instructions are supposed to be telling you how to cd into the npm folder and locally update the copy/copies of node-gyp installed there. The use of the -g flag is inappropriate and leads to npm retaining its old/outdated copy of node-gyp.
• Also, it seems to be necessary (in my testing) to upgrade node-gyp in two places, as multiple packages directly depend on node-gyp; not only npm itself, but the subdependency npm-lifecycle. One must cd into each package and upgrade its node-gyp dependency manually.

[cclauss]

Thanks massively for your precision in fixing our docs.

[DeeDeeG]

Tested again reading/copy-pasting straight from the wiki page, and I can confirm it is working as written. Thank you very much!

Test scenarios:

• Ubuntu 20.04, Node/npm installed with n, in a non-root-protected area (no sudo needed for this case)… Instructions work as written for npm 6 and 7, respectively ✔️
• Windows 10, with one copy of Node/npm installed under the protected “Program Files” dir, another copy installed in a non-protected area by extracting a .zip archive… Instructions work as written for npm 6 and 7, respectively. ✔️

Idle thoughts (no action needed) regarding sudo or Administrator cmd.exe prompt: Users with Node installed in a non-protected area in Windows won’t need Administrator command prompt, just as Unix users don’t need sudo if their copy of Node is installed in a non root-owned/protected area. I think we explain this well enough in the Windows instructions, even if we don’t say so explicitly. I trust Linux users to figure it out. Using elevated permissions doesn’t hurt in this case, anyhow.