fix(consensus): Update median_timespan() method to align with zcashd implementation

[arya2]

Motivation

This PR fixes a minor mistake in the median_timespan() method below block height 28. The bug does not affect checkpoint validation, and does not affect Mainnet or the default Testnet because Zebra requires checkpoint verification until well past block height 28 for those networks.

https://github.com/zcash/zcash/blob/master/src/pow.cpp#L47-L84

Depends-On: #8475.

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Specifications

Section 7.7.3, page 132 of the specification

Solution

Use the MedianTime of the genesis block as the older_median if there are fewer than PoWAveragingWindow blocks in the relevant chain.

Other changes:

• Fixes a typo in dependabot in this PR so CI passes.

Review

Anyone can review, @upbqdn may be interested.

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[upbqdn]

@arya, this PR needs a manual conflict resolution.