github/issue_template: don't use markdown for "Important Information"

There are too many issues created with problematic contents (like "#1"
in uname output) that cause unwanted expansion to markdown formats
and links to issues. The information here should be fairly simple in
format that markdown isn't needed. Use plain text rendering instead.

.github/ISSUE_TEMPLATE/2_bug_report_linux.yml

@@ -46,6 +46,7 @@ body:
           - Window Manager and Version:
           - Source mpv:
           - Introduced in version:
+      render: text
   validations:
       required: true
 - type: textarea

.github/ISSUE_TEMPLATE/2_bug_report_macos.yml

@@ -39,6 +39,7 @@ body:
           - macOS version:
           - Source of mpv:
           - Introduced in version:
+      render: text
   validations:
       required: true
 - type: textarea

.github/ISSUE_TEMPLATE/2_bug_report_windows.yml

@@ -42,6 +42,7 @@ body:
           - GPU model, driver and version:
           - Source of mpv:
           - Introduced in version:
+      render: text
   validations:
       required: true
 - type: textarea

.github/ISSUE_TEMPLATE/3_bug_report.yml

@@ -39,6 +39,7 @@ body:
           - GPU model, driver and version:
           - Source:
           - Introduced in version:
+      render: text
   validations:
       required: true
 - type: textarea

.github/ISSUE_TEMPLATE/4_bug_report_build.yml

@@ -16,6 +16,7 @@ body:
           - mpv version:
           - Platform version:
           - Meson version:
+      render: text
   validations:
       required: true
 - type: textarea