bump minor version to 3.28.0

[zarirhamza]

What does this PR do?

Release bump from v3.27.0 to v3.28.0. This release packages up the Ruby 4.0 work merged in #148.

Notable changes since v3.27.0:

• feat: support Ruby 4.0 (feat: support Ruby 4.0 #148) — adds Datadog-Ruby4-0 and Datadog-Ruby4-0-ARM published layers; bumps bundled datadog gem from 2.12 to 2.30 (required for Ruby 4.0 install support — affects 3.2 / 3.3 / 3.4 layers too); bumps rake dev dependency from ~> 12.3 to ~> 13.0 (Ruby 4.0 removed ostruct from default gems).

Motivation

APMSVLS-512. Cuts a release so the Datadog-Ruby4-0 / Datadog-Ruby4-0-ARM layers get published, which in turn unblocks the Ruby 4.0 e2e tests in serverless-e2e-tests#223.

Testing Guidelines

Per the Lambda Layer Ruby runbook: once this PR's gitlab pipeline finishes its build layer / check layer size / unit test / integration test jobs, the sandbox publish jobs are manual — clicking those will publish test layers to the serverless-sandbox AWS account (425362996713) for verification.

Plan:

1. Wait for build + integration tests to pass.
2. Manually trigger the relevant publish layer sandbox jobs to get test layer ARNs.
3. Run the full ruby34 + ruby40 e2e suite locally from serverless-e2e-tests using those test ARNs (via DD_RUBY_34_ARN / DD_RUBY_40_ARN env vars) before merging.
4. Only after step 3 is green: merge, tag v3.28.0, push tag, click the prod signing + publish jobs per runbook.

This PR is intentionally a draft until step 3 is green.

Additional Notes

Diff is a single-line minor bump of MINOR = 27 → MINOR = 28 in lib/datadog/lambda/version.rb. No code or test changes — 47057fb (the merge of #148) is already on main.

Types of changes

• Bug fix
• New feature
• Breaking change
• Misc (docs, refactoring, dependency upgrade, etc.)

Check all that apply

• This PR's description is comprehensive
• This PR contains breaking changes that are documented in the description
• This PR introduces new APIs or parameters that are documented and unlikely to change in the foreseeable future
• This PR impacts documentation, and it has been updated (or a ticket has been logged)
• This PR's changes are covered by the automated tests
• This PR collects user input/sensitive content into Datadog