Fix completion signature detection with GCC 13.1 and 13.2 - #2280

GCC 13.1 and 13.2 still give the class-type completion-signatures template argument a const-qualified type, so they need the existing remove_const_t workaround too. Extend its upper bound to GCC 13.3, where the unqualified check works.

Fixes #2078.

Validation:

• The issue's sender_in reproducer fails on unchanged main with GCC 13.1 and 13.2. With this change, it compiles with -std=c++20 -Wall -Wextra -Werror and an inline_scheduler / then / start_detached pipeline runs successfully.
• GCC 13.3 compiles and runs the same program both before and after the change.
• The existing test.stdexec suite passes with Apple Clang: 622 test cases, 3,522 assertions.