Add new release notes below the Unreleased section of changelogs

[swang392]

What does this PR do?

Changed changelog generation so that the new version and changelog entries are added below the 'Unreleased' section of the changelog

Motivation

In preparation for enforcing manual changelog entries in integrations, the 'Unreleased' section of the changelog should always stay at the top of the file.

Additional Notes

example (with blank Unreleased section):

example (with content in Unreleased section):

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• PR title must be written as a CHANGELOG entry (see why)
• Files changes must correspond to the primary purpose of the PR as described in the title (small unrelated changes should have their own PR)
• PR must have changelog/ and integration/ labels attached
• If the PR doesn't need to be tested during QA, please add a qa/skip-qa label.

[codecov]

Codecov Report

Merging #15332 (abc29ae) into master (30d560d) will increase coverage by 0.00%.
The diff coverage is n/a.

Flag	Coverage Δ
datadog_checks_dev	82.62% <ø> (+0.07%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

[github-actions]

Test Results

    4 files      4 suites   4m 25s ⏱️
424 tests 424 ✔️   0 💤 0 ❌
948 runs  915 ✔️ 33 💤 0 ❌

Results for commit abc29ae.

[github-actions]

Label changelog/Changed was added to this Pull Request, so the next release will bump major version. Please make sure this is a breaking change, or use the changelog/Fixed label.