Simplify "Char" to "char"

Also one case of "String" to "string" in the same file.

Per PR feedback.

Author: stephentoub

src/System.Security.Cryptography.X509Certificates/src/Internal/Cryptography/Helpers.cs

@@ -35,13 +35,13 @@ public static string ToHexStringUpper(this byte[] bytes)
 
         // Decode a hex string-encoded byte array passed to various X509 crypto api. The parsing rules are overly forgiving but for compat reasons,
         // they cannot be tightened.
-        public static byte[] DecodeHexString(this String s)
+        public static byte[] DecodeHexString(this string s)
         {
             int whitespaceCount = 0;
 
             for (int i = 0; i < s.Length; i++)
             {
-                if (Char.IsWhiteSpace(s[i]))
+                if (char.IsWhiteSpace(s[i]))
                     whitespaceCount++;
             }
 
@@ -50,7 +50,7 @@ public static byte[] DecodeHexString(this String s)
 
             for (int index = 0, i = 0; index < cbHex; index++)
             {
-                if (Char.IsWhiteSpace(s[i]))
+                if (char.IsWhiteSpace(s[i]))
                 {
                     i++;
                 }

src/System.Security.Cryptography.X509Certificates/src/Internal/Cryptography/Pal.Windows/StorePal.Find.cs

@@ -449,7 +449,7 @@ private static void ValidateOidValue(String keyValue)
             for (int i = 1; i < len; i++)
             {
                 // ensure every character is either a digit or a dot
-                if (Char.IsDigit(keyValue[i]))
+                if (char.IsDigit(keyValue[i]))
                     continue;
                 if (keyValue[i] != '.' || keyValue[i + 1] == '.') // disallow double dots
                     throw new ArgumentException(SR.Argument_InvalidOidValue);