Update issue templates (2)

Author: flobernd

.github/ISSUE_TEMPLATE/bug-report-for-version-6-x-and-below.md

@@ -2,7 +2,8 @@
 name: Bug report for version 6.x and below
 about: File a bug report for NEST / Elasticsearch.Net version 6.x and below
 title: ''
-labels: 'Category: Bug'
+labels: 
+  - 'Category: Bug'
 assignees: ''
 
 ---

.github/ISSUE_TEMPLATE/bug-report-for-version-7-x.md

@@ -2,7 +2,9 @@
 name: Bug report for version 7.x
 about: File a bug report for NEST / Elasticsearch.Net version 7.x
 title: ''
-labels: 'Category: Bug', 7.x
+labels: 
+  - 'Category: Bug'
+  - '7.x'
 assignees: ''
 
 ---

.github/ISSUE_TEMPLATE/bug-report-for-version-8-x.md

@@ -2,7 +2,9 @@
 name: Bug report for version 8.x
 about: File a bug report for Elastic.Clients.Elasticsearch version 8.x
 title: ''
-labels: 'Category: Bug', 8.x
+labels: 
+  - 'Category: Bug'
+  - '8.x'
 assignees: ''
 
 ---