feat(self-hosted): convert experimental vars to config options

[RahulGautamSingh]

Changes

• Converted the following experimental env vars to self-hosted config options

Env var	Config option
RENOVATE_X_DOCKER_MAX_PAGES	dockerMaxPages
RENOVATE_X_GITLAB_AUTO_MERGEABLE_CHECK_ATTEMPS	gitlabAutoMergeableCheckAttempts
RENOVATE_X_GITLAB_BRANCH_STATUS_DELAY	gitlabBranchStatusDelay
RENOVATE_X_GITLAB_MERGE_REQUEST_DELAY	gitlabMergeRequestDelay
RENOVATE_X_PLATFORM_VERSION	platformVersion

Context

Part of: #27879 (comment)

Documentation (please check one with an [x])

• I have updated the documentation, or
• No documentation update is required

How I've tested my work (please select one)

I have verified these changes via:

• Code inspection only, or
• Newly added/modified unit tests, or
• No unit tests but ran on a real repository, or
• Both unit tests + ran on a real repository

[RahulGautamSingh]

Verified each migration locally, by logging the values of each option at runtime.
check 1: default values passed to functions ☑️
check 2: custom values, overwrites default values ☑️
check 3: old env vars parsed and then overwrite the default values ☑️

[viceice]

@rarkins @secustor I think we should unify this under a generic platform options object, so we've a single config option with known values. not is best time to do it.

WDYT?

[rarkins]

I'm not sure I understand. Do you mean nesting it inside a platformOptions object? Does it give any advantage other than making sure all possible ones are located together within config?