use `input` for spoiler to save space in rendered form

.gitea/issue_template/proposal.yaml

@@ -14,17 +14,13 @@ body:
       placeholder: stofl, inspired by https://codeforces.com/problemset/problem/1/A
     validations:
       required: true
-  - type: textarea
+  - type: input
     id: spoiler-begin
     attributes:
       label: Start of spoiler
       description: Opening the spoiler tag
-      placeholder: |
-        <details>
-        <summary>Topic Tags</summary>
-      value: |
-        <details>
-        <summary>Topic Tags</summary>
+      placeholder: <details><summary>Topic Tags</summary>
+      value: <details><summary>Topic Tags</summary>
   - type: dropdown
     id: round
     attributes:
@@ -121,7 +117,7 @@ body:
         - warmup
     validations:
       required: false
-  - type: textarea
+  - type: input
     id: spoiler-end
     attributes:
       label: End of spoiler