build: Build the pigweed_default target in C++20

Include the full pigweed_default target in C++20 builds, rather than
manually specifying groups. This gives more coverage (perf test were
missing previously) and ensures the C++20 build stays healthy as
pigweed_default grows.

Change-Id: I7c6f5e21e0ce84a586a721cb6451b3b8b1cef3d7
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/126459
Reviewed-by: Erik Gilling <[email protected]>
Commit-Queue: Auto-Submit <[email protected]>
Pigweed-Auto-Submit: Wyatt Hepler <[email protected]>

BUILD.gn

@@ -393,10 +393,7 @@ group("cpp14_compatibility") {
 # enabled than without.
 group("cpp20_compatibility") {
   _cpp20_tc = "$_internal_toolchains:pw_strict_host_clang_size_optimized_cpp20"
-  deps = [
-    ":apps($_cpp20_tc)",
-    ":pw_module_tests.run($_cpp20_tc)",
-  ]
+  deps = [ ":pigweed_default($_cpp20_tc)" ]
 }
 
 group("build_with_pw_minimal_cpp_stdlib") {