Use workspace-level rust-version

[seadanda]

Workspace fails to compile with a rust version lower than 1.81 due to dependency requirements - probably best to lift the rust-version to the workspace instead of being set in many different files.

It's likely that some packages could compile with a lower version, but just a matter of time before that advances too. Having it in one place makes it easier to maintain.

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I went through the workspace members and found the following rust versions to be the minimum for the default features:

members = [
	"fixed-hash", # 1.64
	"keccak-hash", # 1.79
	"kvdb", # 1.64
	"kvdb-memorydb", # 1.64
	"kvdb-rocksdb", # 1.71.1
	"kvdb-shared-tests", # 1.64
	"parity-bytes", # 1.64
	"rlp", # 1.64
	"rlp-derive", # 1.64
	"uint", # 1.64
	"primitive-types", # 1.79
	"bounded-collections", # 1.79
	"ethereum-types", # 1.79
	"ethbloom", # 1.64
]

1.64 is the minimum for the whole workspace since before this the cargo manifest requires author to be a different format. weirdly for the individual packages I don't see the requirement for 1.81, so I wonder if there are some unused deps at the workspace level or only when certain features are enabled

Originally posted by @seadanda in https://github.com/paritytech/parity-common/pull/906#discussion_r1995416703