docs: [AdsDataManager] describe additional URI format for kek_uri in GcpEncryptionInfo and AwsKmsEncryptionInfo (#8888)

* docs: describe additional URI format for kek_uri in GcpEncryptionInfo and AwsKmsEncryptionInfo
fix!: update `go_package` packaging option from `google.golang.org/genproto/googleapis/ads/datamanager/v1;datamanager` to `cloud.google.com/go/datamanager/apiv1/datamanagerpb;datamanagerpb`
chore: update importpath in BUILD.bazel for datamanager_go_proto
chore: remove extra_opts from datamanager_csharp_proto in BUILD.bazel
chore: update copyright year to 2026

PiperOrigin-RevId: 863287879

Source-Link: googleapis/googleapis@fa3c2e8

Source-Link: googleapis/googleapis-gen@cf4b86a
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