Refactor ExchangeRate class for improved readability and consistency

- Updated class definition and method signatures to follow PSR standards.
- Changed constructor and method implementations to use braces on the same line.
- Improved type hinting and return types for methods.
- Simplified string manipulation and array handling in methods.
- Enhanced code clarity by adding spaces around operators and conditions.
- Minor adjustments in index.php for consistent spacing in conditional statements.

Author: Vítězslav Dvořák

.php-cs-fixer.dist.php

@@ -15,7 +15,7 @@
 
 use Ergebnis\PhpCsFixer\Config\Factory;
 use Ergebnis\PhpCsFixer\Config\Rules;
-use Ergebnis\PhpCsFixer\Config\RuleSet\Php74;
+use Ergebnis\PhpCsFixer\Config\RuleSet\Php81;
 
 $header = <<<'HEADER'
 This file is part of the CNBCache package
@@ -28,7 +28,7 @@
 file that was distributed with this source code.
 HEADER;
 
-$ruleSet = Php74::create()->withHeader($header)->withRules(Rules::fromArray([
+$ruleSet = Php81::create()->withHeader($header)->withRules(Rules::fromArray([
     'blank_line_before_statement' => [
         'statements' => [
             'break',