MDL-67316 typo3: Pass correct hex chars to hexdec()

Typo3 was relaying on the feature of base converter
functions silently removing invalid chars so, for example:

'U+00A0' => '00A0' => 160

Since php74, the existence of those invalid chars do produce
a deprecation warning, no matter the outcome continues being the same.

So, here, we are just converting that invalud 'U+' by '0x'

lib/typo3/class.t3lib_cs.php

@@ -1313,12 +1313,12 @@ function initUnicodeData($mode = NULL) {
 					array_push($code_decomp, chr($ord));
 				}
 			}
-			$ascii[$this->UnumberToChar(hexdec($from))] = join('', $code_decomp);
+			$ascii[$this->UnumberToChar(hexdec(str_replace('U+', '0x', $from)))] = join('', $code_decomp);
 		}
 
 			// add numeric decompositions
 		foreach ($number as $from => $to) {
-			$utf8_char = $this->UnumberToChar(hexdec($from));
+			$utf8_char = $this->UnumberToChar(hexdec(str_replace('U+', '0x', $from)));
 			if (!isset($ascii[$utf8_char])) {
 				$ascii[$utf8_char] = $to;
 			}

lib/typo3/readme_moodle.txt

@@ -10,6 +10,13 @@ Procedure:
 
 Local changes (to verify/apply with new imports):
 
+- MDL-67316: PHP 7.4 compatibility. Wrong chars in hexdec() operations.
+    Ensure that all the calls to hexdec() are passing exclusively
+    correct hex chars. Before php74 they were silently discarded but
+    with php74 a deprecation warning is produced. We haven't looked how
+    this is fixed upstream because plans include to remove this
+    library from core (see MDL-65809)
+
 - MDL-67017: PHP 7.4 compatibility. Curly brackets.
     Remove all the deprecated curly bracket uses {} to access to strings/arrays
     by key. We haven't looked how this is fixed upstream because plans include