Update librustzcash to 6.1

**What**

Update current reference for librustzcash to 6.1

**Why**

6.1 includes new security, performance and functionality updates

**How**

Either by referencing 6.1 then refactoring current implementations around it or by forking 6.1 and refactoring it to be compatible with current implementations.

**Acceptance Criteria**

6.1 referenced and used in webZjs

**Estimation**

@sqhell feel free to add your own estimation

**Dependencies**

-

**References**
[Architecture](https://hackmd.io/@wollum/BJaqaB81kl)
[Slack Thread](https://chainsafeio.slack.com/archives/C06M9F7FJ2K/p1764855960620599)
[Problem](https://docs.google.com/document/d/1HzeQ_TI7sxH5LA9y-HheZ8aCs-iMpPZMTtcUBeQdkTw/edit?pli=1&tab=t.0)
[Upgrade Grant](https://docs.google.com/document/d/1MAm0gpl5oM5dfIeA2GFc5hEkz6Ftg8pi-BTwLFnbcAg/edit?tab=t.0)
@sqhell feel free to add your own additional references