Convert recent RSA key files in PEM format from PKCS8 to PKCS1

Like `openssl rsa`, `openssl genrsa` changed its output format from PKCS8 to
PKCS1 in OpenSSL 3.0. Note that the makefile instructions assume older
OpenSSL. Convert the files that were generated with OpenSSL 3.x and hence
were not in the intended format. The files are converted, not regenerated,
so the key material is the same.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/tests/data_files/Makefile b/tests/data_files/Makefile
index fa30cf5..bbbfa9c 100644
--- a/tests/data_files/Makefile
+++ b/tests/data_files/Makefile
@@ -709,6 +709,8 @@
 ### Basic unencrypted RSA keys from which
 ### all other encrypted RSA keys are derived.
 keys_rsa_base =
+### TODO: the commands require OpenSSL 1.x to work as desired. With
+### OpenSSL 3.x, they produce pkcs8 files.
 rsa_pkcs1_768_clear.pem:
 	$(OPENSSL) genrsa -out $@ 768
 keys_rsa_base += rsa_pkcs1_768_clear.pem