feat: [Grafeas] A new field `last_vulnerability_update_time` is added to message `.grafeas.v1.DiscoveryOccurrence` #6192

Job	Run time
Package Tests (Lowest Dependencies)	9m 53s
Dev Commands Unit Tests	52s
Package Tests	9m 50s
PHP 8.3 Unit Test (ubuntu-latest)	5m 54s
PHP 8.5 Unit Test (ubuntu-latest)	5m 40s
PHP 8.4 Unit Test (ubuntu-latest)	4m 47s
PHP 8.5 Unit Test (windows-latest)	4m 54s
PHP 8.2 Unit Test (ubuntu-latest)	6m 33s
PHP 8.1 Unit Test (ubuntu-latest)	6m 24s
PHP 8.4 Unit Test (windows-latest)	5m 25s
PHP 8.1 Unit Test + protobuf extension (ubuntu-latest)	5m 31s
PHP 8.3 Unit Test (windows-latest)	5m 32s
PHP 8.2 Unit Test (windows-latest)	7m 25s
PHP 8.1 Unit Test (windows-latest)	6m 31s
	1h 25m 11s

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