fix!: remove deprecated pull policy variable (#710)

## Description

Removes the earlier deprecated `runners_pull_policy` variable. Since were making a Major release I thought this one was nice to catch.

## Migrations required

YES. Replace the `runners_pull_policy` by `runners_pull_policies`.

Author: tmeijn

Diff for: README.md

@@ -685,7 +685,6 @@ Made with [contributors-img](https://contrib.rocks).
 | <a name="input_runners_pre_clone_script"></a> [runners\_pre\_clone\_script](#input\_runners\_pre\_clone\_script) | Commands to be executed on the Runner before cloning the Git repository. this can be used to adjust the Git client configuration first, for example. | `string` | `"\"\""` | no |
 | <a name="input_runners_privileged"></a> [runners\_privileged](#input\_runners\_privileged) | Runners will run in privileged mode, will be used in the runner config.toml | `bool` | `true` | no |
 | <a name="input_runners_pull_policies"></a> [runners\_pull\_policies](#input\_runners\_pull\_policies) | pull policies for the runners, will be used in the runner config.toml, for Gitlab Runner >= 13.8, see https://docs.gitlab.com/runner/executors/docker.html#using-multiple-pull-policies | `list(string)` | <pre>[<br>  "always"<br>]</pre> | no |
-| <a name="input_runners_pull_policy"></a> [runners\_pull\_policy](#input\_runners\_pull\_policy) | Deprecated! Use runners\_pull\_policies instead. pull\_policy for the runners, will be used in the runner config.toml | `string` | `""` | no |
 | <a name="input_runners_request_concurrency"></a> [runners\_request\_concurrency](#input\_runners\_request\_concurrency) | Limit number of concurrent requests for new jobs from GitLab (default 1). | `number` | `1` | no |
 | <a name="input_runners_request_spot_instance"></a> [runners\_request\_spot\_instance](#input\_runners\_request\_spot\_instance) | Whether or not to request spot instances via docker-machine | `bool` | `true` | no |
 | <a name="input_runners_root_size"></a> [runners\_root\_size](#input\_runners\_root\_size) | Runner instance root size in GB. | `number` | `16` | no |

Diff for: locals.tf

@@ -78,7 +78,7 @@ locals {
     }
   )
 
-  runners_pull_policies = var.runners_pull_policy != "" ? "[\"${var.runners_pull_policy}\"]" : "[\"${join("\",\"", var.runners_pull_policies)}\"]"
+  runners_pull_policies = "[\"${join("\",\"", var.runners_pull_policies)}\"]"
 
   /* determines if the docker machine executable adds the Name tag automatically (versions >= 0.16.2) */
   # make sure to skip pre-release stuff in the semver by ignoring everything after "-"

Diff for: variables.tf

@@ -266,12 +266,6 @@ variable "runners_helper_image" {
   default     = ""
 }
 
-variable "runners_pull_policy" {
-  description = "Deprecated! Use runners_pull_policies instead. pull_policy for the runners, will be used in the runner config.toml"
-  type        = string
-  default     = ""
-}
-
 variable "runners_pull_policies" {
   description = "pull policies for the runners, will be used in the runner config.toml, for Gitlab Runner >= 13.8, see https://docs.gitlab.com/runner/executors/docker.html#using-multiple-pull-policies "
   type        = list(string)