stellar/go module: deprecation comments for rename to go-stellar-sdk

[sreuland]

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

Added deprecation comments to go module and docs

Why

to handle the renaming of both the git repo and the associated go module from stellar/go to stellar/go-stellar-sdk.

Known limitations

[sreuland]

after merge confirmed the deprecation in go tooling:

$ GOPROXY=direct go get github.com/stellar/go
go: downloading github.com/stellar/go v0.0.0-20251125023735-e21b38861c9c
go: module github.com/stellar/go is deprecated: Use github.com/stellar/go-stellar-sdk instead
go: upgraded github.com/stellar/go v0.0.0-20241104220238-749abd7a661f => v0.0.0-20251125023735-e21b38861c9c