Replace MBEDTLS_ERR_OID_NOT_FOUND with MBEDTLS_ERR_X509_UNKNOWN_OID

Replace the non-X.509-named error code `MBEDTLS_ERR_OID_NOT_FOUND` with
`MBEDTLS_ERR_X509_UNKNOWN_OID`, which already exists and is currently not
used for anything.

Public functions in X.509 propagate this error code, so it needs to have a
public name.

Remove the definition of `MBEDTLS_ERR_OID_NOT_FOUND` in `x509_oid.h`, then

```
git grep -l MBEDTLS_ERR_OID_NOT_FOUND | xargs perl -i -pe 's/\bMBEDTLS_ERR_OID_NOT_FOUND\b/MBEDTLS_ERR_X509_UNKNOWN_OID/g'
```

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/library/ssl_tls13_generic.c b/library/ssl_tls13_generic.c
index 70175e0..44525dd 100644
--- a/library/ssl_tls13_generic.c
+++ b/library/ssl_tls13_generic.c
@@ -505,7 +505,7 @@
         switch (ret) {
             case 0: /*ok*/
                 break;
-            case MBEDTLS_ERR_OID_NOT_FOUND:
+            case MBEDTLS_ERR_X509_UNKNOWN_OID:
                 /* Ignore certificate with an unknown algorithm: maybe a
                    prior certificate was already trusted. */
                 break;