exp/services/ledgerexporter/internal: Fix adjustLedgerRange()

[tamirms]

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

adjustLedgerRange() attempts to clamp the start and end ledger for a given ledger range so that they align on the ledgers per file boundary. However, there is a bug in the implementation because the end ledger should be aligned to the last ledger in the file.

For example, if we attempt to export ledgers in the range of [1, 50] assuming 64 ledgers per file, the adjusted range should be [2, 63] based on this logic:

https://github.com/stellar/go/blob/master/exp/services/ledgerexporter/internal/exportmanager.go#L54-L64

The current code instead adjusts the range to [2, 64] which is incorrect because 64 is the start ledger of the next ledger file

Known limitations

[N/A]