(textproc/R-stringi) Updated 1.7.4 to 1.7.6

# What Is New in *stringi*

## 1.7.6 (2021-11-29)

* [BUILD TIME] #463: Added loongarch support in ICU's double conversion
    (@liuxiang88).

* [BUGFIX] #467: The UCRT build on Windows was not marking strings as `latin1`.

## 1.7.5 (2021-10-04)

* [DOCUMENTATION] Paper on *stringi* has been accepted for
  publication in the *Journal of Statistical Software*,
  see <https://stringi.gagolewski.com/_static/vignette/stringi.pdf>
  for a draft version.

* [DOCUMENTATION] The *stringi* website at <https://stringi.gagolewski.com>
  now features a comprehensive tutorial based on the aforementioned paper.

* [DOCUMENTATION] The *ICU* Project site has been moved to
  <https://icu.unicode.org/>.

* [BUILD TIME] #457: The `autoconf` macros `AC_LANG_CPLUSPLUS`
  and `AC_TRY_COMPILE` were obsolete.

* [BUGFIX] #458: Passing ALTREP objects no longer yields
  'embeded nul in string' errors.

(mef)