Fix nullness RFC link in issue template (#18369)

Author: MangelMaxime

.github/ISSUE_TEMPLATE/nullness_issue.yml

@@ -13,7 +13,7 @@ body:
   - type: markdown
     attributes:
       value: |
-        Please check at [Nullable Reference Types RFC](https://github.com/fsharp/fslang-design/blob/main/RFCs/FS-1060-nullable-reference-types.md) if this issue isn't a known limitation, such as missing flow-control analysis after branching constructs like `if` or `while`. Also check at [existing nullness issues](https://github.com/dotnet/fsharp/issues?q=is%3Aissue+label%3AArea-Nullness+) if it hasn't been reported already.
+        Please check at [Nullable Reference Types RFC](https://github.com/fsharp/fslang-design/blob/main/FSharp-9.0/FS-1060-nullable-reference-types.md) if this issue isn't a known limitation, such as missing flow-control analysis after branching constructs like `if` or `while`. Also check at [existing nullness issues](https://github.com/dotnet/fsharp/issues?q=is%3Aissue+label%3AArea-Nullness+) if it hasn't been reported already.
         Please provide a succinct description of the issue and choose 1 or more from the following categories of impact
   - type: checkboxes
     id: categories
@@ -73,4 +73,4 @@ body:
         Are there any language constructs (typically pattern matching, library constructs, Null/NonNull active patterns, explicit type annotations) allowing the same logic be expressed differently in order to mitigate the experienced nullness issue?
       placeholder: Possible workaround
     validations:
-      required: false
+      required: false