🔖 set version to 3.12.0

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

.github/ISSUE_TEMPLATE/bug.yaml

@@ -76,7 +76,7 @@ body:
       label: Library version
       description: >
         Which version of the library did you use? If it is a released version,
-        please enter the version number (e.g., 3.11.3). Otherwise, please enter
+        please enter the version number (e.g., 3.12.0). Otherwise, please enter
         the commit hash. If you got the library from another source as the
         GitHub repository (e.g., via a package manager), please also state
         this.