git: change submodule tracking from master to main. (#4934)

Khronos is master branched on SPIRV repos to main. We should start
tracking it now.

external/SPIRV-Headers

@@ -1 +1 @@
-Subproject commit 34d04647d384e0aed037e7a2662a655fc39841bb
+Subproject commit d13b52222c39a7e9a401b44646f0ca3a640fbd47

external/SPIRV-Tools

@@ -1 +1 @@
-Subproject commit d87f61605b3647fbceae9aaa922fce0031afdc63
+Subproject commit 7e8813bb4cf32fd9c4d696c09d4e68e17a8f57f1