Allow lockFileMaintenance PRs to not be blocked by concurrent branch limits

[jamietanna]

Tell us more.

When using Renovate's Lock File Maintenance (aka lockFileMaintenance functionality - which we enabled by default in Renovate 43 - a possible occurrence is that Renovate has too many branches open to be able to raise a Lock File Maintenance PR.

Because Lock File Maintenance is an option that generally keeps dependencies in-sync with the version pinning (or ranges) that are in use, this is likely something that users always want to have raised for them.

However, due to it not receiving any priority over the other branches created, it may get rate limited by branchConcurrentLimits/prConcurrentLimits.

We probably don't want to make this a default (at the lockFileMaintenance configuration option) but instead introduce a new preset which can then be ignorePresets'd.

For instance:

{
  "$schema": "https://docs.renovatebot.com/renovate-schema.json",
  "packageRules": [
    {
      "description": "Always allow lockFileMaintenance to be raised",
      "matchUpdateTypes": [
        "lockFileMaintenance"
      ],
      "branchConcurrentLimit": 0
    }
  ]
}

[RahulGautamSingh]

Sounds good 👍🏻