Fix jsonschema validation warning for docker services_limit

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What does this MR do?

This MR fixes a jsonschema validation warning for the docker services_limit option.

It is based on the solutions from !4248 (merged) (and !4724 (merged)) and adds the omitempty flag to the ServicesLimit option inside common/config.go. There is already a default/fallback logic in place that sets this value to -1 in case no other value is provided by the user.

Why was this MR needed?

Running gitlab-runner list with a basic config.toml currently throws a jsonschema validation warning:

$ gitlab-runner list
Runtime platform                                    arch=amd64 os=linux pid=100411 revision=81ab07f6 version=16.10.0
There might be a problem with your config based on jsonschema annotations in common/config.go (experimental feature):
jsonschema: '/runners/0/docker/services_limit' does not validate with https://gitlab.com/gitlab-org/gitlab-runner/common/config#/$ref/properties/runners/items/$ref/properties/docker/$ref/properties/services_limit/type: expected integer, but got null

Listing configured runners                          ConfigFile=/etc/gitlab-runner/config.toml
runner-host-001                                     Executor=docker Token=xj484ijsdjoiv892 URL=https://gitlab.company.local

What's the best way to test this MR?

To reproduce the issue:

1. Create a basic config.toml that uses the docker executor (see example config below)
2. Download the latest gitlab-runner binary
3. Run gitlab-runner list --config path/to/config.toml
4. Verify that the validation warning is being displayed

To test the fix:

1. Re-use the config.toml
2. Build the gitlab-runner binary
3. Run gitlab-runner list --config path/to/config.toml
4. Ensure that no validation warnings are being displayed

Example `config.toml`

listen_address = ":9252"
concurrent = 3
check_interval = 0
connection_max_age = "15m0s"
shutdown_timeout = 0

[session_server]
  session_timeout = 1800

[[runners]]
  name = "runner-host-001"
  output_limit = 409600
  url = "https://git.company.local"
  id = 901
  token = "xj484ijsdjoiv892"
  token_obtained_at = 2024-04-09T07:03:53Z
  token_expires_at = 0001-01-01T00:00:00Z
  executor = "docker"
  environment = ["DOCKER_TLS_CERTDIR=/certs"]
  shell = "bash"
  [runners.cache]
    Type = "s3"
    Shared = true
    MaxUploadedArchiveSize = 0
    [runners.cache.s3]
      ServerAddress = "minio.company.local"
      AccessKey = "access-key"
      SecretKey = "secret-key"
      BucketName = "gitlab-runner"
      AuthenticationType = "access-key"
  [runners.feature_flags]
    FF_USE_IMPROVED_URL_MASKING = true
  [runners.docker]
    tls_verify = false
    image = "registry.company.local/docker/docker:stable"
    privileged = true
    disable_entrypoint_overwrite = false
    oom_kill_disable = false
    disable_cache = false
    volumes = ["/cache", "/certs/client"]
    pull_policy = ["always"]
    shm_size = 2147483648
    network_mtu = 0

What are the relevant issue numbers?

Fixes Warning message about services_limit. (#37424 - closed)