Revert rust-version increasing rewrite

Author: 197g

static-alloc/src/unsync/bump.rs

@@ -251,7 +251,10 @@ impl MemBump {
     /// This is how many *bytes* can be allocated
     /// within this node.
     pub const fn capacity(&self) -> usize {
-        self.data.get().len()
+        // Safety: just gets the pointer metadata `len` without invalidating any provenance,
+        // accepting the pointer use itself. This may be replaced by a safe `pointer::len` as soon
+        // as stable (#71146) and const which would avoid any pointer use.
+        unsafe { (*(self.data.get() as *const [UnsafeCell<u8>])).len() }
     }
 
     /// Get a raw pointer to the data.