[icu] Upgrades the ICU library version in use

This change brings in ICU 70.1 which is the latest and greatest of ICU releases.
It sync up with the latest ICU milestone in use in Chromium.

Issue: #flutter/97412

Author: filmil

DEPS

@@ -146,7 +146,7 @@ deps = {
    # Chromium-style dependencies.
 
   'src/third_party/icu':
-   Var('chromium_git') + '/chromium/deps/icu.git' + '@' + '2b50fa94b07b601293d7c1f791e853bba8ffbb84',
+   Var('chromium_git') + '/chromium/deps/icu.git' + '@' + '609143a639dcc0884a87c899614895ce129afdb8',
 
   'src/third_party/khronos':
    Var('chromium_git') + '/chromium/src/third_party/khronos.git' + '@' + '7122230e90547962e0f0c627f62eeed3c701f275',