services/ledgerexporter: Move ledgerexporter out of experimental

[urvisavla]

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

1. Move ledgerexporter from the experimental directory to the main services directory.
2. Rename Ledgerexporter to Galexie.

Why

https://stellarorg.atlassian.net/browse/HUBBLE-456

Known limitations

We will need to rename the Docker repository created for ledgerexporter and update the metrics dashboard with the new prefix.

[sreuland]

looks good, after seeing the move/rename pattern, I skimmed through remainder, doesn't look like any new funcationality. good call-out on the need for docker repo rename as well.