Fix Style/NegatedUnless offenses in HAML file

Signed-off-by: Rémy Coutable <remy@rymai.me>

app/views/clusters/platforms/kubernetes/_form.html.haml

 = bootstrap_form_for cluster, url: update_cluster_url_path, html: { class: 'gl-show-field-errors' },
   as: :cluster do |field|
   - copy_name_btn = clipboard_button(text: cluster.name, title: s_('ClusterIntegration|Copy Kubernetes cluster name'),
-    class: 'input-group-text btn-default') unless !cluster.read_only_kubernetes_platform_fields?
+    class: 'input-group-text btn-default') if cluster.read_only_kubernetes_platform_fields?
   = field.text_field :name, class: 'js-select-on-focus cluster-name', required: true,
   title: s_('ClusterIntegration|Cluster name is required.'),
   readonly: cluster.read_only_kubernetes_platform_fields?,
@@ -10,7 +10,7 @@
 
   = field.fields_for :platform_kubernetes, platform do |platform_field|
     - copy_api_url = clipboard_button(text: platform.api_url, title: s_('ClusterIntegration|Copy API URL'),
-    class: 'input-group-text btn-default') unless !cluster.read_only_kubernetes_platform_fields?
+    class: 'input-group-text btn-default') if cluster.read_only_kubernetes_platform_fields?
     = platform_field.text_field :api_url, class: 'js-select-on-focus', required: true,
     title: s_('ClusterIntegration|API URL should be a valid http/https url.'),
     readonly: cluster.read_only_kubernetes_platform_fields?,
@@ -18,7 +18,7 @@
     input_group_class: 'gl-field-error-anchor', append: copy_api_url
 
     - copy_ca_cert_btn = clipboard_button(text: platform.ca_cert, title: s_('ClusterIntegration|Copy CA Certificate'),
-      class: 'input-group-text btn-default') unless !cluster.read_only_kubernetes_platform_fields?
+      class: 'input-group-text btn-default') if cluster.read_only_kubernetes_platform_fields?
     = platform_field.text_area :ca_cert, class: 'js-select-on-focus', rows: '5',
     readonly: cluster.read_only_kubernetes_platform_fields?,
     placeholder: s_('ClusterIntegration|Certificate Authority bundle (PEM format)'),
@@ -28,7 +28,7 @@
     - show_token_btn = (platform_field.button s_('ClusterIntegration|Show'),
       type: 'button', class: 'js-show-cluster-token btn btn-default')
     - copy_token_btn = clipboard_button(text: platform.token, title: s_('ClusterIntegration|Copy Service Token'),
-      class: 'input-group-text btn-default') unless !cluster.read_only_kubernetes_platform_fields?
+      class: 'input-group-text btn-default') if cluster.read_only_kubernetes_platform_fields?
 
     = platform_field.text_field :token, type: 'password', class: 'js-select-on-focus js-cluster-token',
     required: true, title: s_('ClusterIntegration|Service token is required.'),