Bump MSRV to 1.81 (#582)

This is required due to the `hybrid-array` crate, which has become a
transitive dependency of the majority of our dependencies and will be
required in the very near future.

src/error.rs

@@ -192,8 +192,8 @@ impl std::error::Error for Error {
     }
 }
 
-impl From<x509_cert::der::Error> for Error {
-    fn from(_err: x509_cert::der::Error) -> Error {
+impl From<der::Error> for Error {
+    fn from(_err: der::Error) -> Error {
         Error::ParseError
     }
 }