Replace Openssl3 with Modern2023, make Modern equal to Modern2023

OpenSSL 3 and Java 20 use the same algorithms by default, but differ in
the number of KDF iterations and the KDF salt length.  This suggests that
there won't be any compatibility problems if I diverge from OpenSSL's
salt length.  Therefore, Modern2023 uses a salt length of 16.

pkcs12.go

@@ -124,34 +124,39 @@ var Passwordless = &Encoder{
 	rand:          rand.Reader,
 }
 
-// Openssl3 encodes PKCS#12 files using OpenSSL 3's default parameters.
-// Private keys and certificates are encrypted using PBES2 with PBKDF2
-// (2048 iterations of HMAC-SHA-2) and AES-256-CBC.  The MAC algorithm is HMAC-SHA-2.
-var Openssl3 = &Encoder{
+// Modern2023 encodes PKCS#12 files using algorithms that are considered modern
+// as of 2023.  Private keys and certificates are encrypted using PBES2 with
+// PBKDF2-HMAC-SHA-256 and AES-256-CBC.  The MAC algorithm is HMAC-SHA-2.  These
+// are the same algorithms used by OpenSSL 3 (by default), Java 20 (by default),
+// and Windows Server 2019 (when "stronger" is used).
+//
+// Files produced with this encoder can be read by OpenSSL 1.1.1 and higher,
+// Java 12 and higher, and Windows Server 2019 and higher.
+//
+// For passwords, it is RECOMMENDED that you do one of the following:
+// 1) Use [DefaultPassword] and protect the file using other means, or
+// 2) Use a high-entropy password, such as one generated with `openssl rand -hex 16`.
+//
+// You SHOULD NOT use a lower-entropy password with this encoder because the number of KDF
+// iterations is only 2048 and doesn't provide meaningful protection against
+// brute forcing.  You can increase the number of iterations using [Encoder.WithIterations],
+// but as https://neilmadden.blog/2023/01/09/on-pbkdf2-iterations/ explains, this doesn't
+// help as much as you think.
+var Modern2023 = &Encoder{
 	macAlgorithm:         oidSHA256,
 	certAlgorithm:        oidPBES2,
 	keyAlgorithm:         oidPBES2,
 	macIterations:        2048,
 	encryptionIterations: 2048,
-	saltLen:              8,
+	saltLen:              16,
 	rand:                 rand.Reader,
 }
 
-// Modern encodes PKCS#12 files using modern, secure parameters.
-// Private keys and certificates are encrypted using PBES2 with PBKDF2
-// (600,000 iterations of HMAC-SHA-2) and AES-256-CBC.  The MAC algorithm is HMAC-SHA-2.
+// Modern encodes PKCS#12 files using modern, robust parameters.
 //
-// The behavior of this encoder may change in backwards-incompatible ways
-// to keep up with modern practices.
-var Modern = &Encoder{
-	macAlgorithm:         oidSHA256,
-	certAlgorithm:        oidPBES2,
-	keyAlgorithm:         oidPBES2,
-	macIterations:        600000,
-	encryptionIterations: 600000,
-	saltLen:              16,
-	rand:                 rand.Reader,
-}
+// Currently, this encoder is the same as [Modern2023], but this
+// may change in the future to keep up with modern practices.
+var Modern = Modern2023
 
 var (
 	oidDataContentType          = asn1.ObjectIdentifier([]int{1, 2, 840, 113549, 1, 7, 1})