Adyen: Update API version

Summary:
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This PR changes the PAYMENT_API_VERSION updating it to a newer version. According to changelog and previous update there
is not any additional requirements to handle the update

Test Execution:
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Unit test
Finished in 0.06828 seconds.
94 tests, 479 assertions, 0 failures, 0 errors, 0 pendings, 0 omissions, 0 notifications
100% passed

Remote test
Finished in 163.203064 seconds.
123 tests, 439 assertions, 3 failures, 0 errors, 0 pendings, 0 omissions, 0 notifications
97.561% passed
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Failures not related with the added code

RuboCop:
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739 files inspected, no offenses detected

lib/active_merchant/billing/gateways/adyen.rb

@@ -17,7 +17,7 @@ class AdyenGateway < Gateway
       self.homepage_url = 'https://www.adyen.com/'
       self.display_name = 'Adyen'
 
-      PAYMENT_API_VERSION = 'v64'
+      PAYMENT_API_VERSION = 'v68'
       RECURRING_API_VERSION = 'v49'
 
       STANDARD_ERROR_CODE_MAPPING = {