[abseil] Upgrade to 20200923 patch 3, fix gcc11 build error (#16664)

ports/abseil/portfile.cmake

@@ -5,8 +5,8 @@ endif()
 vcpkg_from_github(
     OUT_SOURCE_PATH SOURCE_PATH
     REPO abseil/abseil-cpp
-    REF 0f3bb466b868b523cf1dc9b2aaaed65c77b28862 #LTS 20200923, Patch 2
-    SHA512 17e766a2f7a655a3877eb3accc5745a910b69a5e2426b7ce7f6d31095523dd32d48a709c5f8380488b4cb93ce9faadedc08f0481dbdbd00cf68831541d724b4d
+    REF 6f9d96a1f41439ac172ee2ef7ccd8edf0e5d068c #LTS 20200923, Patch 3
+    SHA512 f64fee62863f2103c1991136fd3bc2b71cd28c7ff62138ac991b5a7f81780a05e0e2bdd6a119d02e1d70dd54f989f584093957efaec94f26c9d6c3f4ee37f8ae
     HEAD_REF master
     PATCHES
         # in C++17 mode, use std::any, std::optional, std::string_view, std::variant

ports/abseil/vcpkg.json

@@ -1,7 +1,7 @@
 {
   "name": "abseil",
   "version-string": "2020-09-23",
-  "port-version": 2,
+  "port-version": 3,
   "description": [
     "an open-source collection designed to augment the C++ standard library.",
     "Abseil is an open-source collection of C++ library code designed to augment the C++ standard library. The Abseil library code is collected from Google's own C++ code base, has been extensively tested and used in production, and is the same code we depend on in our daily coding lives.",

versions/a-/abseil.json

@@ -1,5 +1,10 @@
 {
   "versions": [
+    {
+      "git-tree": "184230b7622b1be5630f893dcecdaac7acde101f",
+      "version-string": "2020-09-23",
+      "port-version": 3
+    },
     {
       "git-tree": "1558fdc71dd0f0988cd2a53ba439bed3cc899247",
       "version-string": "2020-09-23",

versions/baseline.json

@@ -10,7 +10,7 @@
     },
     "abseil": {
       "baseline": "2020-09-23",
-      "port-version": 2
+      "port-version": 3
     },
     "absent": {
       "baseline": "0.3.1",