Update CHANGELOG for Horizon release 2.31.0

[aditya1702]

PR Checklist

PR Structure

• This PR has reasonably narrow scope (if not, break it down into smaller PRs).
• This PR avoids mixing refactoring changes with feature changes (split into two PRs
  otherwise).
• This PR's title starts with name of package that is most changed in the PR, ex.
  services/friendbot, or all or doc if the changes are broad or impact many
  packages.

Thoroughness

• This PR adds tests for the most critical parts of the new functionality or fixes.
• I've updated any docs (developer docs, .md
  files, etc... affected by this change). Take a look in the docs folder for a given service,
  like this one.

Release planning

• I've updated the relevant CHANGELOG (here for Horizon) if
  needed with deprecations, added features, breaking changes, and DB schema changes.
• I've decided if this PR requires a new major/minor version according to
  semver, or if it's mainly a patch change. The PR is targeted at the next
  release branch if it's not a patch change.

What

Update Changelog for the release.

Why

N/A

Known limitations

N/A

[sreuland]

lgtm

[urvisavla]

I think we should mention in the changelog that EXPERIMENTAL_BUCKETLIST_DB=true must be set in the captive-core config. 89068a9.

[sreuland]

I think we should mention in the changelog that EXPERIMENTAL_BUCKETLIST_DB=true must be set in the captive-core config. 89068a9.

I think this conditional case is still automated in toml.go, i.e. it will add EXPERIMENTAL_BUCKETLIST_DB=true when absent and UseDB = true

https://github.com/aditya1702/go/blob/changelog/ingest/ledgerbackend/toml.go#L541

but, I think we should mention the opposite use case when using a custom captive core config and Horizon UseDB=false, user needs to ensure that they remove EXPERIMENTAL_BUCKETLIST_DB from captive core config toml when stellar-core is less than 21.0.0 and if they are using stellar-core 21.0.0. or up, then should add:

EXPERIMENTAL_BUCKETLIST_DB = false

Once #5333 merges, then that simplifies this, removes the need to do EXPERIMENTAL_BUCKETLIST_DB = false anymore.