Update dependency MediatR to v12.2.0 #252

Job	Run time
people-service-dotnet-scan	1m 0s
account-service-gradle-scan	1m 10s
reference-data-node-scan	59s
database-gradle-scan	48s
trade-processor-dockerfile-scan	1m 43s
trade-feed-node-scan	30s
position-service-gradle-scan	1m 6s
database-dockerfile-scan	1m 27s
web-front-end/angular-node-scan	1m 9s
trade-processor-gradle-scan	1m 2s
account-service-dockerfile-scan	1m 47s
web-front-end/proxy-node-scan	28s
trade-service-gradle-scan	58s
trade-feed-dockerfile-scan	1m 41s
web-front-end/react-node-scan	1m 51s
position-service-dockerfile-scan	1m 43s
trade-service-dockerfile-scan	1m 42s
web-front-end/angular-dockerfile-scan	3m 26s
people-service-dockerfile-scan	1m 20s
reference-data-dockerfile-scan	2m 51s
	28m 41s

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