Switch to file-per-change changelog (#3774)

## Motivation and Context
Implements [RFC: File-per-change
changelog](https://smithy-lang.github.io/smithy-rs/design/rfcs/rfc0042_file_per_change_changelog.html#rfc-file-per-change-changelog)

## Description
When this PR is merged to `main`, it will impact developer workflow for
creating a changelog entry that goes with a PR.
Prior to this, one edited `CHANGELOG.next.toml` to enter a changelog
entry, but it often caused merge conflicts among us. With this PR, one
creates a `.md` file (any filename stem will do) under the
`smithy-rs/.changelog` directory. This Markdown file has the YAML front
matter, and it can be created manually or via the `ChangeLogger` CLI
(see [this
commit](https://github.com/smithy-lang/smithy-rs/pull/3774/commits/83cda766089f8d47401d3693f21791e34c770595)
for more details).

Here are the highlights of the code changes:
- [The release
script](https://github.com/smithy-lang/smithy-rs/pull/3774/files#diff-162dc2b82eb3d72b105291d2fc5fc8d10c4fbc1e6f35862bc08ac0d6a7c53f76)
has been pointed to `smithy-rs/.changelog`. This is pretty much what it
takes to let the `smithy-rs` release infra switch to the new changelog
mode.
-
[sdk-lints](https://github.com/smithy-lang/smithy-rs/pull/3774/commits/efcc772b37940c9e0f676e6eedac134ae5ad044e)
now disallows `CHANGELOG.next.toml`. It will complain if the file is
present.
- The `--source-to-truncate` option for the `changelogger`'s `render` is
now optional because our internal release infra code no longer needs to
specify that option (i.e. there is nothing to truncate there).
- Tests in `smithy-rs-tool-common` and `changelogger` have been updated
so TOML changelog entries used in tests have been re-written to Markdown
changelog entries. We want to avoid giving the false impression that
TOML changelog entries are still well-supported.

## Testing
- Existing tests & lints in CI
- Ran `smithy-rs` [dry-run
release](https://github.com/smithy-lang/smithy-rs/actions/runs/10100442953)
and confirmed the correctness of `CHANGELOG.md` and
`SDK_CHANGELOG.next.json` in the release artifacts
- Ran the internal release and confirmed `CHANGELOG.md` was rendered
correctly in `aws-sdk-rust`

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