Use network_config over network in google_alloydb_cluster examples

[rileykarson]

Current examples all use network, we should use the non-deprecated variant in all of them (or all but one if we want an example of the old mechanism to stick around). Related to #16119, the removal in 6.0.0.

We shouldn't eliminate usages entirely, and should keep some in handwritten tests.

b/319230983

[c2thorn]

Note that this is heavily related to #16119 and should be done simultaneously

[c2thorn]

closing as duplicate of #16119

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