ICU-20979 Update TODOs in formatting code to point to open issues.

Also see: ICU-20920 ICU-21059 ICU-20429 ICU-21058
diff --git a/icu4c/source/i18n/measunit_extra.cpp b/icu4c/source/i18n/measunit_extra.cpp
index c978565..8af9e41 100644
--- a/icu4c/source/i18n/measunit_extra.cpp
+++ b/icu4c/source/i18n/measunit_extra.cpp
@@ -92,7 +92,7 @@
     { "yocto", UMEASURE_SI_PREFIX_YOCTO },
 };
 
-// TODO(ICU-20920): Get this list from data
+// TODO(ICU-21059): Get this list from data
 const char16_t* const gSimpleUnits[] = {
     u"one", // note: expected to be index 0
     u"candela",
diff --git a/icu4c/source/test/depstest/dependencies.txt b/icu4c/source/test/depstest/dependencies.txt
index e096586..022b144 100644
--- a/icu4c/source/test/depstest/dependencies.txt
+++ b/icu4c/source/test/depstest/dependencies.txt
@@ -958,7 +958,7 @@
 
 group: number_representation
     number_decimalquantity.o string_segment.o number_utils.o
-    # TODO(ICU-20429) Move formatted_string_builder to its own unit.
+    # TODO(ICU-21058) Move formatted_string_builder to its own unit.
     formatted_string_builder.o
   deps
     decnumber double_conversion