[ci.yml, rush.json] Add Node 18, drop Node 14

[mikeharder]

Summary

• Add Node 18 to supported and tested versions
• Remove Node 14 since it is EOL
• Node 20 cannot be tested yet, since Node 20 is only supported by pnpm@>=8.3.1 ("ERR_INVALID_THIS" on "pnpm up" in Node 20 pnpm/pnpm#6424), so the rushstack repo would need to upgrade pnpm in rush.json

How it was tested

The CI run for this PR should validate itself.

Impacted documentation

None

[mikeharder]

Node 18 is failing some scenarios with this error:

--[ FAILURE: hashed-folder-copy-plugin-webpack4-test (build) ][ 4.17 seconds ]--

node:internal/crypto/hash:71
  this[kHandle] = new _Hash(algorithm, xofLen);
                  ^

Error: error:0308010C:digital envelope routines::unsupported
    at new Hash (node:internal/crypto/hash:71:19)
    at Object.createHash (node:crypto:133:10)
    at module.exports (/home/runner/work/rushstack/rushstack/common/temp/node_modules/.pnpm/[email protected]/node_modules/webpack/lib/util/createHash.js:135:53)
    at NormalModule._initBuildHash (/home/runner/work/rushstack/rushstack/common/temp/node_modules/.pnpm/[email protected]/node_modules/webpack/lib/NormalModule.js:417:16)
    at handleParseError (/home/runner/work/rushstack/rushstack/common/temp/node_modules/.pnpm/[email protected]/node_modules/webpack/lib/NormalModule.js:471:10)
  ...2 lines omitted...
    at /home/runner/work/rushstack/rushstack/common/temp/node_modules/.pnpm/[email protected]/node_modules/loader-runner/lib/LoaderRunner.js:373:3
    at iterateNormalLoaders (/home/runner/work/rushstack/rushstack/common/temp/node_modules/.pnpm/[email protected]/node_modules/loader-runner/lib/LoaderRunner.js:214:10)
    at Array.<anonymous> (/home/runner/work/rushstack/rushstack/common/temp/node_modules/.pnpm/[email protected]/node_modules/loader-runner/lib/LoaderRunner.js:205:4) {
  opensslErrorStack: [ 'error:03000086:digital envelope routines::initialization error' ],
  library: 'digital envelope routines',
  reason: 'unsupported',
  code: 'ERR_OSSL_EVP_UNSUPPORTED'
}

[dmichon-msft]

Webpack 4 is not currently compatible with Node 18; this is a known issue. We'd need to either get fixes/workarounds into Webpack 4 or figure out how to disable the webpack 4 projects when running with Node 18.
@TheLarkInn for thoughts on Webpack 4.

[mikeharder]

@dmichon-msft: I think it would also be nice to add Node 20, but this would require the rushstack repo to upgrade to pnpm@>=8.3.1.

[iclanton]

Handled by #4330 and #4278