Bump version to 28.1.0

[github-actions]

What

Bump version to 28.1.0, creating release branch.

Why

Triggered by @fnando in https://github.com/stellar/stellar-cli/actions/runs/36190058803.

What is next

See the release instructions for a full rundown on the release process:
https://github.com/stellar/actions/blob/main/README-rust-release.md

Commit any changes to the release/v28.1.0 branch that are needed in this release.

If this is a regular release releasing from main, merge this PR when ready, and after merging, create a release for this version by going to this link: https://github.com/stellar/stellar-cli/releases/new?tag=v28.1.0&title=28.1.0

If this is a backport or patch release of a past version, see the release instructions. When ready to release this branch create a release by going to this link:
https://github.com/stellar/stellar-cli/releases/new?tag=v28.1.0&title=28.1.0&target=release/v28.1.0

[socket-security]

Review the following changes in direct dependencies. Learn more about Socket for GitHub.

Diff	Package	Supply Chain Security	Vulnerability	Quality	Maintenance	License
	cargo/​soroban-sdk@​28.0.0-rc.1 ⏵ 28.0.0
	cargo/​soroban-spec@​28.0.0-rc.1 ⏵ 28.0.0
	cargo/​soroban-spec-rust@​28.0.0-rc.1 ⏵ 28.0.0
	cargo/​soroban-ledger-snapshot@​28.0.0-rc.1 ⏵ 28.0.0
	cargo/​soroban-token-sdk@​28.0.0-rc.1 ⏵ 28.0.0
	cargo/​stellar-asset-spec@​28.0.0-rc.1 ⏵ 28.0.0

View full report