Merge to tee-dev but with v0.9.26 rollback

[Kailai-Wang]

Context

As I experimented with dependency update for TEE-worker (as I wanted to test the shielding-key ocall), it seems we have problems right now to update both to v0.9.28:

• use parachain 0.9.28 + tee-worker 0.9.26 won't work, at least not so simple: Request of updating the rust toolchain version integritee-network/worker#1000
• update worker to 0.9.28 is blocked by Polkadot upgrade v0.9.28 integritee-network/worker#992 (comment)

To not block the dev process, I guess the only way for now is to merge dev to tee-dev branch, and then rollback the dependency to v0.9.26, so that we have 0.9.26 on both sides.

If you have better ideas, please let me know

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[Kailai-Wang]

Maybe let's hold on for a while, Integritee mentioned v0.9.28 update should be complete soon

I'd still expect that retain_mut problem though, but we can resolve it by patching substrate.

[Kailai-Wang]

superseded by litentry/tee-worker#64