Decide, document, and enforce accepted merge strategies

[BigLep]

Done Criteria

1. We have documentation for those with merge permissions about what merge strategies we accept/use and when.
2. We disable any merge strategies we don't want to allow.

Why Important

Inconsistencies or mis-wielding here was a source of pain in the nv23/1.28.0 release train, especially when wanting to compare branches to ensure the right commits were present. Some places this came up:

1. https://filecoinproject.slack.com/archives/CP50PPW2X/p1719274557199449?thread_ts=1718923493.254039&cid=CP50PPW2X
2. https://github.com/filecoin-project/lotus/pull/12138
3. https://filecoinproject.slack.com/archives/CP50PPW2X/p1719287930380079

This also affects the general tidiness of the commit history which can impact ease which one can quickly grok the progression of changes.

User/Customer

Maintainers but also users who inspect the commit history.

Notes

1. At least of 2024-08-26, Lotus supports merge commits, squash merging, and rebase merging:
2. We have a separate issue concerning a standard process for backporting changes: https://github.com/filecoin-project/lotus/issues/12418

## Tasks
- [ ] PR with any relevant updates to RELEASE_FLOW and RELEASE_TEMPLATE based on https://github.com/filecoin-project/lotus/issues/12417#issuecomment-2335046166
- [ ] Lotus repo updates to disable merge commits