Update issue templates

Author: flobernd

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

@@ -2,7 +2,7 @@
 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: Bug
+labels: 'Category: Bug'
 assignees: ''
 
 ---

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

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

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

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

.github/ISSUE_TEMPLATE/feature_request.md

@@ -2,7 +2,7 @@
 name: Feature request
 about: Suggest a feature
 title: ''
-labels: Feature
+labels: 'Category: Feature'
 assignees: ''
 
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