Browser warnings related to `macaron_flash` cookie

[silverwind]

Description

I observe these during development in Firefox, I think we should somehow fix these:

Gitea Version

main

Can you reproduce the bug on the Gitea demo site?

Yes

[pierre-eliep-met]

I get the same warning, and additionnaly I can't create a Pull request on my project, don't know if it is related

[wxiaoguang]

I get the same warning, and additionnaly I can't create a Pull request on my project, don't know if it is related

100% unrelated 😁

If you see some errors or incorrect logs, please open a new issue with details.

[wxiaoguang]

The cookie problem: the FlashMessage code keeps sending "deleting cookie" header to browsers.

Eventually .... Refactor cookie #24107 because the old cookie code doesn't look good to me and has bugs .....