Allow specifying "maximum" values for user-configurable values in self-hosting config

[jamietanna]

There are a number of repo config options that can have significant impact to the runtime of the CLI, depending on how it's deployed.

For instance, running the CLI in autodiscovery mode, or using another "single-threaded" option can mean that the longer a single repository takes, the longer the whole execution of the CLI (against all repos) takes, overall.

For configuration such as prConcurrentLimit and branchConcurrentLimit, which are very useful for tuning how many updates a given repository can have.

However, when running self-hosted, this can lead to significant impact if you have a big repository with a lot of dependencies, resulting in lots of branches being created by Renovate, possibly taking up time that other repos will not be able to process for (i.e. if running Mend Renovate Community Self-Hosted, or based on Mend Renovate Cloud limits)

This feature would:

• allow setting global self-hosting configuration (i.e. limits.branchConcurrentLimit or maximumLimits.branchConcurrentLimit) to provide a maximum limit that this value can be at
• a DEBUG (or a WARN?) would be logged when overriding this compared to a repo's config

This could also include hostRules.concurrentRequestLimit

(As discussed in #39433)