bootutil: Unify app_max_size() implementations

Remove redundant application size calculations in favor of
a swap-specific function, implemented inside swap_<type>.c.
In this way, slot sizes use the same restrictions as image validation.

Note: This PR has been modified from the upstream commit due to
a merge failure from other changes that are not being brought into
Zephyr 3.7

Signed-off-by: Tomasz Chyrowicz <tomasz.chyrowicz@nordicsemi.no>
(cherry picked from commit f9db0194)