Remove offpeak settings docker autoscaling

What does this MR do?

Deprecate the OffPeak field from runner

Why was this MR needed?

This MR is needed to remove the support of the Offpeak configuration from runner. The details of the chosen implementation to achieve it, can be seen here

What's the best way to test this MR?

Two different outcomes must be tested:

Absence of Offpeak fields when registering a new docker+machine runner

1. Register a new docker+machine with the following command

gitlab-runner register \
    --non-interactive \ 
    --executor "docker+machine" \ 
    --docker-image "alpine:3.11.7" \ 
    --url "https://gitlab.com" \ 
    --description "Docker Machine Runner" \ 
    --registration-token "REGISTRATION_TOKEN_HERE"

The config.toml generated should be free of any Offpeak fields as below

Generated config.toml

concurrent = 1
check_interval = 0

[session_server]
  session_timeout = 1800

[[runners]]
  name = "Docker Machine Runner"
  url = "https://gitlab.com"
  token = "RUNNER_TOKEN_HERE"
  executor = "docker+machine"
  [runners.custom_build_dir]
  [runners.cache]
    [runners.cache.s3]
    [runners.cache.gcs]
    [runners.cache.azure]
  [runners.docker]
    tls_verify = false
    image = "alpine:3.11.7"
    privileged = false
    disable_entrypoint_overwrite = false
    oom_kill_disable = false
    disable_cache = false
    volumes = ["/cache"]
    shm_size = 0
  [runners.machine]
    IdleCount = 0
    MachineDriver = ""
    MachineName = ""

Deprecation log for Offpeak configuration when sets

config.toml to use for this test

concurrent = 1
check_interval = 0

[session_server]
  session_timeout = 1800

[[runners]]
  name = "Local GitLab Runner for tests and debugging"
  url = "https://gitlab.com"
  token = "YOUR_TOKEN_HERE"
  executor = "docker+machine"
  [runners.custom_build_dir]
  [runners.cache]
    [runners.cache.s3]
    [runners.cache.gcs]
    [runners.cache.azure]
  [runners.docker]
    tls_verify = false
    image = "alpine:3.11.7"
    privileged = false
    disable_entrypoint_overwrite = false
    oom_kill_disable = false
    disable_cache = false
    volumes = ["/cache"]
    shm_size = 0
  [runners.machine]
    IdleCount = 0
    MachineDriver = "virtualbox"
    MachineName = "offpeak-test-%s"
    OffPeakPeriods = ["*", "*", "3-4", "*", "*", "*", "*"]
    OffPeakTimezone = ""
    OffPeakIdleCount = 2
    OffPeakIdleTime = 2000

Once the runner launched, the following warning message will be logged :

WARNING: OffPeak docker machine configuration is deprecated and has been removed since 14.0. Please use convert the setting into a [[docker.machine.autoscaling]] configuration instead: https://docs.gitlab.com/runner/configuration/autoscale.html#off-peak-time-mode-configuration-deprecated

The Off Peak time mode configuration (Deprecated) section of the configuration/autoscale.html page has been updated to give indication on how to manually convert in [[runners.machine.autoscaling]]

What are the relevant issue numbers?

closes: #25555 (closed)