Merge bitcoin/bitcoin#27064: system: use %LOCALAPPDATA% as default datadir on windows

84900ac34f doc: add release-notes-27064.md (Matthew Zipkin)
855dd8d592 system: use %LOCALAPPDATA% as default datadir on windows (Matthew Zipkin)

Pull request description:

  Closes https://github.com/bitcoin/bitcoin/issues/2391

  This PR changes the default datadir location on Windows from `C:\Users\Username\AppData\Roaming\Bitcoin` to `C:\Users\Username\AppData\Local\Bitcoin`. This change only applies to fresh installs. To preserve backwards compatibility, on startup we check for the existence of `C:\Users\Username\AppData\Roaming\Bitcoin\chainstate` and if it is there, we continue using the "Roaming" directory as the default datadir location.

  [Note that in Windows 11 this change may be moot:](https://learn.microsoft.com/en-us/uwp/api/windows.storage.applicationdata.roamingfolder?view=winrt-22621)

  > Roaming data and settings is no longer supported as of Windows 11. The recommended replacement is [Azure App Service](https://learn.microsoft.com/en-us/azure/app-service/). Azure App Service is widely supported, well documented, reliable, and supports cross-platform/cross-ecosystem scenarios such as iOS, Android and web. Settings stored here no longer roam (as of Windows 11), but the settings store is still available.

ACKs for top commit:
  achow101:
    ACK 84900ac34f
  BenWestgate:
    crACK 84900ac34f
  hebasto:
    re-ACK 84900ac34f, only addressed feedback since my recent [review](https://github.com/bitcoin/bitcoin/pull/27064#pullrequestreview-2028718273).

Tree-SHA512: 807c6e89571287e2c8f4934229aec91ef28e7d0a675234acf1b7d085c24c7b73a08b6e345fbfc9038e6239187b6b69c08490ddaa1c057de5ea975c4a000bba42

src/common/args.cpp

@@ -696,12 +696,19 @@ bool HasTestOption(const ArgsManager& args, const std::string& test_option)
 
 fs::path GetDefaultDataDir()
 {
-    // Windows: C:\Users\Username\AppData\Roaming\Bitcoin
+    // Windows:
+    //   old: C:\Users\Username\AppData\Roaming\Bitcoin
+    //   new: C:\Users\Username\AppData\Local\Bitcoin
     // macOS: ~/Library/Application Support/Bitcoin
     // Unix-like: ~/.bitcoin
 #ifdef WIN32
     // Windows
-    return GetSpecialFolderPath(CSIDL_APPDATA) / "Bitcoin";
+    // Check for existence of datadir in old location and keep it there
+    fs::path legacy_path = GetSpecialFolderPath(CSIDL_APPDATA) / "Bitcoin";
+    if (fs::exists(legacy_path)) return legacy_path;
+
+    // Otherwise, fresh installs can start in the new, "proper" location
+    return GetSpecialFolderPath(CSIDL_LOCAL_APPDATA) / "Bitcoin";
 #else
     fs::path pathRet;
     char* pszHome = getenv("HOME");