clients/horizonclient: replace github.com/manucorporat/sse package with github.com/gin-contrib/sse

[hazzik]

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 reviewed the changes in this PR and if I consider them worthwhile for being mentioned on release notes then I have updated the relevant CHANGELOG.md within the component folder structure. For example, if I changed horizon, then I updated (services/horizon/CHANGELOG.md. I add a new line item describing the change and reference to this PR. If I don't update a CHANGELOG, I acknowledge this PR's change may not be mentioned in future release notes.
• 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

Replace github.com/manucorporat/sse package with more recent widely accepted fork github.com/gin-contrib/sse.

Why

The original package was last updated 10 years ago.

Known limitations

N/A

[socket-security]

New and updated dependencies detected. Learn more about Socket for GitHub ↗︎

Package	New capabilities	Transitives	Size	Publisher
golang/github.com/gin-contrib/sse@v1.0.0	eval, filesystem, network, unsafe	0	26.2 kB
golang/github.com/stretchr/testify@v1.9.0 ➜ v1.10.0	None	0	641 kB

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