Use secure cookie for HTTPS sites (#26999)

If the AppURL(ROOT_URL) is an HTTPS URL, then the COOKIE_SECURE's
default value should be true.

And, if a user visits an "http" site with "https" AppURL, they won't be
able to login, and they should have been warned. The only problem is
that the "language" can't be set either in such case, while I think it
is not a serious problem, and it could be fixed easily if needed.

![image](https://github.com/go-gitea/gitea/assets/2114189/7bc9a859-dcc1-467d-bc7c-1dd6a10389e3)

docs/content/administration/config-cheat-sheet.en-us.md

@@ -777,7 +777,7 @@ and
 
 - `PROVIDER`: **memory**: Session engine provider \[memory, file, redis, redis-cluster, db, mysql, couchbase, memcache, postgres\]. Setting `db` will reuse the configuration in `[database]`
 - `PROVIDER_CONFIG`: **data/sessions**: For file, the root path; for db, empty (database config will be used); for others, the connection string. Relative paths will be made absolute against _`AppWorkPath`_.
-- `COOKIE_SECURE`: **false**: Enable this to force using HTTPS for all session access.
+- `COOKIE_SECURE`:**_empty_**: `true` or `false`. Enable this to force using HTTPS for all session access. If not set, it defaults to `true` if the ROOT_URL is an HTTPS URL.
 - `COOKIE_NAME`: **i\_like\_gitea**: The name of the cookie used for the session ID.
 - `GC_INTERVAL_TIME`: **86400**: GC interval in seconds.
 - `SESSION_LIFE_TIME`: **86400**: Session life time in seconds, default is 86400 (1 day)