[flang][docs] Tweak in-progress warning in the release notes page

Reformat the in-progress warning message in the release notes page. This
is now rendered in a style similar to that of clang. The warning will
be hidden on release builds of the documentation.

Author: tarunprabhu

flang/docs/ReleaseNotes.md

@@ -3,11 +3,13 @@ ReleaseNotes.md and ReleaseNotesTemplate.txt. -->
 
 # Flang {{version}} {{in_progress}}Release Notes
 
-> **warning**
->
-> These are in-progress notes for the upcoming LLVM {{version}} release.
-> Release notes for previous releases can be found on [the Download
-> Page](https://releases.llvm.org/download.html).
+````{only} PreRelease
+```{warning}
+ These are in-progress notes for the upcoming LLVM {{version}} release.
+ Release notes for previous releases can be found on [the Download
+ Page](https://releases.llvm.org/download.html).
+```
+````
 
 ## Introduction
 

flang/docs/ReleaseNotesTemplate.txt

@@ -3,11 +3,13 @@ ReleaseNotes.md and ReleaseNotesTemplate.txt. -->
 
 # Flang {{version}} {{in_progress}}Release Notes
 
-> **warning**
->
-> These are in-progress notes for the upcoming LLVM {{version}} release.
-> Release notes for previous releases can be found on [the Download
-> Page](https://releases.llvm.org/download.html).
+````{only} PreRelease
+```{warning}
+These are in-progress notes for the upcoming LLVM {{version}} release.
+Release notes for previous releases can be found on [the Download
+Page](https://releases.llvm.org/download.html).
+```
+````
 
 ## Introduction