go.mod: rotate go 1.19 from supported versions

[eliottness]

What does this PR do?

As per the go supported version policy. We are rotating go 1.19 from the supported versions in favor of go 1.22.

• Replace go 1.19 with go 1.20 for job running the oldest supported version
• Rotate go 1.19 in favor of go 1.22 for jobs running all supported versions via a matrix
• Modify the README to declare that go 1.19 is now legacy
• Upgrade go mod version
• Upgrade benchmarking platform to 1.20 (https://github.com/DataDog/benchmarking-platform/pull/45) and use the new image built by the CI

Fixes #2595

Reviewer's Checklist

• Changed code has unit tests for its functionality at or near 100% coverage.
• System-Tests covering this feature have been added and enabled with the va.b.c-dev version tag.
• 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.
• Add an appropriate team label so this PR gets put in the right place for the release notes.
• Non-trivial go.mod changes, e.g. adding new modules, are reviewed by @DataDog/dd-trace-go-guild.

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: 2024-03-14 12:21:06

Comparing candidate commit 1f1be38 in PR branch eliott.bouhana/deprecate-go-119 with baseline commit a69e46c in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 41 metrics, 0 unstable metrics.

[felixge]

Thank you so much 🙇