Forking mode: support multiple baseBranches

[rarkins]

Describe the proposed change(s).

Today, forking mode only supports the default branch as base branch, and baseBranches does not work.

This is because:

• Each base branch needs to be force updated to match the upstream repo's SHA
• Such updating is done during initRepo using GitHub's API:

  renovate/lib/modules/platform/github/index.ts

  Lines 671 to 685 in 7bfddb4

  	// This is a lovely "hack" by GitHub that lets us force update our fork's default branch
  	// with the base commit from the parent repository
  	const url = `repos/${config.repository}/git/refs/heads/${config.defaultBranch}`;
  	const sha = repo.defaultBranchRef.target.oid;
  	try {
  	logger.debug(
  	`Updating forked repository default sha ${sha} to match upstream`,
  	);
  	await githubApi.patchJson(url, {
  	body: {
  	sha,
  	force: true,
  	},
  	token: coerceString(forkToken, opts.token),
  	});

• During initRepo, Renovate doesn't know what baseBranches there will be

This base branch sync'ing logic should be switched to the git layer, which would also be one less reason stopping support for forking mode in alternative platforms too.

[rarkins]

Previous attempt: #13808

[HonkingGoose]

When you add support for multiple baseBranches you should also update the docs.

Currently the Renovate docs, Forking Renovate app drawbacks section says:

Drawbacks

If you use Forking Renovate, you'll miss out on these features of the regular Renovate app:

• Automerge
• The baseBranches config option

The baseBranches config options docs mention Forking Renovate in a note admonition:

Do not use the baseBranches config option when you've set a forkToken. You may need a forkToken when you're using the Forking Renovate app.