CDRIVER-5844 fix macOS format specifier

[kevinAlbs]

Fixes a build error observed after upgrading from macOS 12 to macOS 13.7.2:

mongoc-secure-transport.c:335:81: error: format specifies type 'int' but the argument has type 'SecExternalItemType' (aka 'unsigned int') [-Werror,-Wformat]
  335 |       MONGOC_ERROR ("Cannot work with keys of type \"%d\". Please file a JIRA", type);
      |                                                      ~~                         ^~~~
      |                                                      %u

SecExternalItemType is represented as uint32_t. I expect the error is triggered by a local upgrade to clang.

Added improvements:

• A string representation of the enum value is added to the error.
• "Please file a JIRA" is replaced with "Type is not supported" for consistency (it was the only error message referencing JIRA).

Verified with this patch build.