meta: replace default blank issue (#4271)

* meta: disable blank issues

* meta: add blank issue template

Author: Flash0ver

.github/ISSUE_TEMPLATE/config.yml

@@ -0,0 +1 @@
+blank_issues_enabled: false

.github/ISSUE_TEMPLATE/maintainer-blank.yml

@@ -0,0 +1,17 @@
+name: Blank Issue
+description: Blank Issue. Reserved for maintainers.
+labels: [".NET"]
+body:
+  - type: textarea
+    id: description
+    attributes:
+      label: Description
+      description: Please describe the issue.
+    validations:
+      required: true
+
+  - type: markdown
+    attributes:
+      value: |-
+        ## Thanks 🙏
+        Check our [triage docs](https://open.sentry.io/triage/) for what to expect next.