Rename "invisible crypto" config setting to something else we agree on

[andybalaam]

### Tasks
- [ ] https://github.com/element-hq/element-x-ios/issues/3322
- [ ] https://github.com/element-hq/element-x-android/issues/3528

[pmaier1]

Proposal

Ignore unverified devices when sending/receiving messages

[richvdh]

The js-sdk exposes a separate mode called "transition mode". Apparently this means:

• Encryption will throw an error if a verified user has an unsigned device, or if a verified user replaces their identity.
• Events are decrypted only if they come from cross-signed devices, or devices that existed before the Rust crypto SDK started tracking device trust: other events will result in a decryption failure.

(See matrix-org/matrix-js-sdk#4407)

Now that I look at that list, I can't see why it would ever be useful. @BillCarsonFr: any ideas?

[richvdh]

We're doing some brainstorming to figure out what we actually want here

[richvdh]

Now that I look at that list, I can't see why it would ever be useful. @BillCarsonFr: any ideas?

After discussion: we believe that we do not need a separate "transition mode" at the application level, but we do need some flexibility at the js-sdk level. We'll need some changes to the current js-sdk impl though: matrix-org/matrix-js-sdk#4428.

[richvdh]

Following a suggestion by @mxandreas, I think we're instead converging on:

Exclude insecure devices when sending/receiving messages

[BillCarsonFr]

Following a suggestion by @mxandreas, I think we're instead converging on:

Exclude insecure devices when sending/receiving messages

Right, will update EIX/EAX side when doing https://github.com/element-hq/crypto-internal/issues/353 and https://github.com/element-hq/crypto-internal/issues/354

[BillCarsonFr]

Following a suggestion by @mxandreas, I think we're instead converging on:

Exclude insecure devices when sending/receiving messages

Right, will update EIX/EAX side when doing element-hq/crypto-internal#353 and element-hq/crypto-internal#354

In review in element-hq/element-x-ios#3358 and element-hq/element-x-android#3569