Replace shim/ with facebook/buck2-shims-meta submodule (#8656)

After the recent move of most things in shim to shim_et, shim/ was mostly an outdated version of buck2-shims-meta. Now it is buck2-shims-meta.

Test Plan: regular GitHub CI on this rev (cmake is still dependent on buck right now)

Author: swolchok

.gitmodules

@@ -70,3 +70,6 @@
 [submodule "third-party/pocketfft"]
 	path = third-party/pocketfft
 	url = https://github.com/mreineck/pocketfft
+[submodule "shim"]
+	path = shim
+	url = https://github.com/facebook/buck2-shims-meta