Allow flexibility in OS type selections in issue forms

GitHub issue forms are used in this repository to facilitate the creation of high quality issues. These provide input
fields for each of the distinct classes of information which will be essential for the evaluation of the issues.

One of these fields is for the user's operating system. A dropdown menu is used for the selection of the high level
operating system type. Previously this only permitted the selection of a single option. A devoted contributor might have
made the effort to determine that the issue applies to multiple operating system types only to be met with the inability
to provide this information via the dedicated field.

The field also did not offer an option to indicate that the operating system was irrelevant to the issue (e.g., a
subject related to the repository assets).

Those issues are resolved by the following changes:

- Configure the field to allow multiple selections
- Add a "N/A" option to the menu

.github/ISSUE_TEMPLATE/bug-report.yml

@@ -37,11 +37,13 @@ body:
     id: os
     attributes:
       label: Operating system
-      description: Which operating system are you using on your computer?
+      description: Which operating system(s) are you using on your computer?
+      multiple: true
       options:
         - Windows
         - Linux
         - macOS
+        - N/A
     validations:
       required: true
   - type: input

.github/ISSUE_TEMPLATE/feature-request.yml

@@ -32,11 +32,13 @@ body:
     id: os
     attributes:
       label: Operating system
-      description: Which operating system are you using on your computer?
+      description: Which operating system(s) are you using on your computer?
+      multiple: true
       options:
         - Windows
         - Linux
         - macOS
+        - N/A
     validations:
       required: true
   - type: input