fix: issue templates used outdated label names (#2991)

.github/ISSUE_TEMPLATE/2-web-bug.yml

@@ -1,6 +1,6 @@
 name: 🌐 Website bug (modrinth.com)
 description: Report an issue on the Modrinth website.
-labels: [bug, frontend]
+labels: [bug, web]
 body:
   - type: checkboxes
     attributes:
@@ -49,4 +49,4 @@ body:
       label: Additional context
       description: Add any other context about the problem here.
     validations:
-      required: false
+      required: false

.github/ISSUE_TEMPLATE/3-api-bug.yml

@@ -1,6 +1,6 @@
 name: 🛠️ API issue (api.modrinth.com)
 description: Report an issue regarding the Modrinth API.
-labels: [bug, api]
+labels: [bug, backend]
 body:
   - type: checkboxes
     attributes:
@@ -38,4 +38,4 @@ body:
       label: Additional context
       description: Add any other context about the problem here.
     validations:
-      required: false
+      required: false