docs(issue template): do not skip closed issues in link to existing issues (#3728)

When an issue has been already handled by our part, and thus gets
closed, but affects many users and the fix takes a while to be rolled
out, it usually happens that those who notice the matter later on don't
notice previous reports and create duplicate issues.

Let's try to improve a little bit on that by not filtering out closed
issues in the links for checking whether the same issue was already
reported before. This should make it more obvious to users who follow
the link whether an issue for their problem already exists.

.github/ISSUE_TEMPLATE/1-app-bug.yml

@@ -6,7 +6,7 @@ body:
     attributes:
       label: Please confirm the following.
       options:
-        - label: I checked the [existing issues](https://github.com/modrinth/code/issues) for duplicate problems
+        - label: I checked the [existing issues](https://github.com/modrinth/code/issues?q=is%3Aissue) for duplicate problems
           required: true
         - label: I have tried resolving the issue using the [support portal](https://support.modrinth.com)
           required: true

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

@@ -6,7 +6,7 @@ body:
     attributes:
       label: Please confirm the following.
       options:
-        - label: I checked the [existing issues](https://github.com/modrinth/code/issues) for duplicate problems
+        - label: I checked the [existing issues](https://github.com/modrinth/code/issues?q=is%3Aissue) for duplicate problems
           required: true
         - label: I have tried resolving the issue using the [support portal](https://support.modrinth.com)
           required: true

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

@@ -6,7 +6,7 @@ body:
     attributes:
       label: Please confirm the following.
       options:
-        - label: I checked the [existing issues](https://github.com/modrinth/code/issues) for duplicate problems
+        - label: I checked the [existing issues](https://github.com/modrinth/code/issues?q=is%3Aissue) for duplicate problems
           required: true
         - label: I have tried resolving the issue using the [support portal](https://support.modrinth.com)
           required: true

.github/ISSUE_TEMPLATE/4-feature-request.yml

@@ -7,7 +7,7 @@ body:
     attributes:
       label: Please confirm the following.
       options:
-        - label: I checked the [existing issues](https://github.com/modrinth/code/issues) for duplicate feature requests
+        - label: I checked the [existing issues](https://github.com/modrinth/code/issues?q=is%3Aissue) for duplicate feature requests
           required: true
         - label: I have checked that this feature request is not on our [roadmap](https://roadmap.modrinth.com)
           required: true