Update some links

Author: DavisVaughan

inst/include/cpp11/altrep.hpp

@@ -2,5 +2,5 @@
 
 // It would be nice to remove this since all supported versions of R have ALTREP, but
 // some groups rely on both this `#define` and `altrep.hpp` itself existing, like arrow:
-// https://github.com/apache/arrow/blob/50f2d6e04e8323119d4dd31506827ee398d6b8e4/r/src/altrep.cpp#L27-L29
+// https://github.com/r-lib/cpp11/issues/413
 #define HAS_ALTREP

inst/include/cpp11/protect.hpp

@@ -16,8 +16,7 @@
 
 // We would like to remove this, since all supported versions of R now support proper
 // unwind protect, but some groups rely on it existing, like arrow and systemfonts
-// https://github.com/r-lib/systemfonts/blob/02b567086379edaca1a9b3620ad6776e6bb876a7/src/utils.h#L11
-// https://github.com/apache/arrow/blob/50f2d6e04e8323119d4dd31506827ee398d6b8e4/r/src/safe-call-into-r-impl.cpp#L49
+// https://github.com/r-lib/cpp11/issues/412
 #define HAS_UNWIND_PROTECT
 
 #ifdef CPP11_USE_FMT