internal/version: bump to v1.56.0-dev

[nsrip-dd]

What does this PR do?

Bumps the version to the next dev minor release version

Motivation

Reviewer's Checklist

• Changed code has unit tests for its functionality at or near 100% coverage.
• There is a benchmark for any new code, or changes to existing code.
• If this interacts with the agent in a new way, a system test has been added.

For Datadog employees:

• If this PR touches code that handles credentials of any kind, such as Datadog API keys, I've requested a review from @DataDog/security-design-and-guidance.
• This PR doesn't touch any of that.

Unsure? Have a question? Request a review!

[pr-commenter]

Benchmarks

Benchmark execution time: 2023-09-07 14:28:42

Comparing candidate commit 690c7e3 in PR branch nick.ripley/bump-dev-version with baseline commit 106b68a in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 40 metrics, 1 unstable metrics.