Add a new synonym for the legacy transaction pool

[matthew1001]

Description

As an enterprise user planning to set up a permissioned chain, I want to be able to choose a TX pool that's more suitable to my requirements, and isn't called legacy which implies it might be deprecated.

Acceptance Criteria

A new transaction pool name for what is currently called the legacy pool is decided, and either legacy or the new name can be set as a value for the --tx-pool configuration parameter.

For maintainers and community members to vote on the most suitable term for the pool, I've added a comment for each option that was discussed in the 11/28/2023 EMEA/AMER contributor call, with a short explanation. People can vote with a thumbs-up on the one they are most happy with and after a week or two I'll raise a PR with the new name.

Note: I've included a couple of "anti-suggestions" so people can see some of the options that were generally ruled out in the contributor call discussion. Thumbs-up on this will be taken to mean people agree that they are not suitable.

I've started the ball rolling with a vote for sequenced.

[matthew1001]

Suggestion 1: sequenced

Reasoning: The layered pool primarily describes its implementation, as opposed to any particular use-case, so sequenced would be following that precedent. Since the legacy pool has had changes recently to make it more FIFO-like in its behaviour (see for example #6106 / #6022) and thus a little "fairer" from the point of view of individual senders, sequenced describes a little about its implementation and about how TX selection is done.

[matthew1001]

Suggestion 2: flat

Reasoning: Similar to comment 1, flat describes a little about its implementation without being specific about any particular use-case.

[matthew1001]

Anti-Suggestion 1: enterprise

Reason for avoiding: It implies that the layered pool isn't suitable for enterprise use-cases, which is not the case.

[matthew1001]

Anti-Suggestion 2: trusted

Reason for avoiding: It implies too much about permissioning-like features.

[fab-10]

sequenced (or FIFO) works well in the case it is used in a gas free network, less if there is a gas market, in which case the main order is dictated by the gas price.
Another use case for this pool, is to be suitable for more throughput, at the cost of lowering some anti-DoS protection (this required some more development), since it is to be used on trusted network

It is not simple to find a name that summarize all these feature, and since I see that the development of this pool should target the use cases of gas free (fairness) and private network (throughput w/o spam protection) I am proposing also a new approach to naming, a kind of acronym that could be expanded to cover the different features.