Raise the MSVC floor to MSVC 2019 (16.0) in accordance with

https://github.com/google/oss-policies-info/blob/main/foundational-cxx-support-matrix.md

PiperOrigin-RevId: 521573177
Change-Id: I1f12ddbd8516314ee2ed7f8ba6a6895b03c6c270

absl/base/policy_checks.h

@@ -44,10 +44,10 @@
 // Toolchain Check
 // -----------------------------------------------------------------------------
 
-// We support Visual Studio 2017 (MSVC++ 15.0) and later.
+// We support Visual Studio 2019 (MSVC++ 16.0) and later.
 // This minimum will go up.
-#if defined(_MSC_VER) && _MSC_VER < 1910 && !defined(__clang__)
-#error "This package requires Visual Studio 2017 (MSVC++ 15.0) or higher."
+#if defined(_MSC_VER) && _MSC_VER < 1920 && !defined(__clang__)
+#error "This package requires Visual Studio 2019 (MSVC++ 16.0) or higher."
 #endif
 
 // We support GCC 7 and later.