pgsql: ICU: do not convert locale 'C' to 'en-US-u-va-posix'.

ICU: do not convert locale 'C' to 'en-US-u-va-posix'. Older versions of ICU canonicalize "C" to "en-US-u-va-posix"; but starting in ICU version 64, the "C" locale is considered obsolete. Postgres commit ea1db8ae70 introduced code to always canonicalize "C" to "en-US-u-va-posix" for consistency and

pgsql: ICU: do not convert locale 'C' to 'en-US-u-va-posix'.

ICU: do not convert locale 'C' to 'en-US-u-va-posix'. The conversion was intended to be for convenience, but it's more likely to be confusing than useful. The user can still directly specify 'en-US-u-va-posix' if desired. Discussion: https://postgr.es/m/f83f089ee1e9acd5dbbbf3353294d24e1f196e95.