fix: deprecate the createScopedRequired methods

[JustinBeckwith]

BREAKING CHANGE: The createScopedRequired method has been deprecated on multiple classes. The createScopedRequired and createScoped methods on the JWT class were largely in place to help inform clients when scopes were required in an application default credential scenario. Instead of checking if scopes are required after creating the client, instead scopes should just be passed either into the GoogleAuth.getClient method, or directly into the JWT constructor.

Old code

auth.getApplicationDefault(function(err, authClient) {
   if (err) {
     return callback(err);
   }
  if (authClient.createScopedRequired && authClient.createScopedRequired()) {
    authClient = authClient.createScoped([
      'https://www.googleapis.com/auth/cloud-platform'
    ]);
  }
  callback(null, authClient);
});

New code

const client = await auth.getClient({
  scopes: ['https://www.googleapis.com/auth/cloud-platform']
});

[codecov-io]

Codecov Report

Merging #410 into master will increase coverage by 0.28%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #410      +/-   ##
==========================================
+ Coverage   94.87%   95.15%   +0.28%     
==========================================
  Files          15       15              
  Lines         956      970      +14     
  Branches      217      217              
==========================================
+ Hits          907      923      +16     
+ Misses         49       47       -2

Impacted Files	Coverage Δ
src/messages.ts	100% <100%> (ø)	⬆️
src/auth/computeclient.ts	95% <100%> (+0.26%)	⬆️
src/auth/iam.ts	100% <100%> (+25%)	⬆️
src/auth/jwtclient.ts	95.04% <100%> (+0.2%)	⬆️
src/auth/jwtaccess.ts	98.18% <100%> (+0.06%)	⬆️

---

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[JustinBeckwith]

gentle ping @google/google-node-team @google/yoshi-nodejs

[ofrobots]

LGTM w/ nits.