Added past audit reports

[rytimx]

Audit Report added under audit/bridge/

• zkEVM Bridge Contract
• Flow rate contract

[drinkcoffee]

I would rename: audits/bridge/flow-rate/202308-immutable-securityreview.pdf to audits/bridge/flow-rate/202308-external-audit-trail-of-bits.pdf

[drinkcoffee]

I would rename: audits/bridge/zkevm/202311-immutable-zkevmbridgecontracts-securityreview.pdf audits/bridge/zkevm/202311-external-audit-train-of-bits-zkevmbridgecontracts.pdf

[drinkcoffee]

That is, YYYYNN-external-audit--name.pdf

[rytimx]

Thanks @drinkcoffee , rename the files as recommended.

[ermyas]

nit: "..train-of-bits" -> "..tail-of-bits"

[ermyas]

should we be using the date on the final audit report instead?

[rytimx]

Thanks @ermyas , fixed the typo.