Use genpkey instead of genrsa to generate private keys

`genrsa` is being deprecated by openssl for a long time and `genpkey` do
the same thing (and more) better so we might as well use it.

CHANGELOG.md

@@ -11,6 +11,8 @@ This project does not follow semantic versioning.
 
 ### Changed
 
+* Use `genpkey` instead of `genrsa` to generate private keys.
+
 ### Deprecated
 
 ### Removed

make-csr.sh

@@ -126,7 +126,7 @@ openssl_key(){
 
     [ -w "${key_dir}" ] || error "Directory ${key_dir} is not writable"
 
-    "${OPENSSL_BIN}" genrsa -out "${key}" "${size}" 2> /dev/null
+    "${OPENSSL_BIN}" genpkey -algorithm RSA -pkeyopt "rsa_keygen_bits:${size}" -out "${key}" 2> /dev/null
 
     [ -r "${key}" ] || error "Something went wrong, ${key} has not been generated"
 }