Do not error on valid format specifications like strprintf("arg2=%2$s arg1=%1$s arg2=%2$s", arg1, arg2);
Some existing doctests need to be modified to use valid format specifiers (%b and %z do not exist)
This is a partial backport of core#27302.
https://github.com/bitcoin/bitcoin/pull/27302/commits/398c3719b02197ad92fded20f6ff83b364747297