Deprecate force_ssl_login()

[johnbillion]

Since [28609], force_ssl_login() is just a wrapper for force_ssl_admin() and isn't used in core.

The FORCE_SSL_LOGIN constant could be deprecated too, but there's no need as it just behaves the same as FORCE_SSL_ADMIN.

[johnbillion]

In 34700:

Deprecate force_ssl_login(), which is simply a wrapper for force_ssl_admin() and is not used in core.

Fixes #34011

[johnbillion]

In 34703:

force_ssl_login() has been functionally identical to force_ssl_admin() since 4.0 (#10267), so its test assertions can be removed as the function has been deprecated in [34700]. The remaining assertions suitably cover force_ssl_admin().

See #34011

[DrewAPicture]

In 34706:

Docs: Add an @see reference for the "use-instead-of" function in the DocBlock for force_ssl_login(), deprecated in [34700].

See #34011. See #32246.