all: align Go support with upstream policy

[nsrip-dd]

What does this PR do?

Align our Go support policy with the official policy of the Go project

Motivation

The Go project supports only the two most recent language versions. See
https://go.dev/doc/devel/release#policy. Older language versions do not
receive bug/security fixes. This commit aligns the dd-trace-go support
policy with the official one, dropping support for the third most recent
language version (1.20 as of this commit). This will let us more quickly
adopt new features from Go, such as structured logging from slog or
runtime execution trace flight recording, or any other new APIs added in
the future.

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.

Unsure? Have a question? Request a review!

[pr-commenter]

Benchmarks

Benchmark execution time: 2024-06-26 14:57:39

Comparing candidate commit 7ba62b2 in PR branch nick.ripley/tighten-go-support-policy with baseline commit b70405a in branch main.

Found 0 performance improvements and 0 performance regressions! Performance is the same for 46 metrics, 1 unstable metrics.

[github-actions]

This PR is stale because it has been open 20 days with no activity. Remove stale label or comment or this will be closed in 10 days.

[darccio]

LGTM, but I think we should be careful with any toolchain directive in our go.mod files.

[darccio]

Suggested change

	toolchain go1.22.2
	toolchain go1.21.11

[eliottness]

Can you remove this if block here ? I also seems mentions to go 1.20 in the CI in multios-unit-tests.yml and in parametric.yml. Can we switch to stable ?

[nsrip-dd]

Can you remove this if block here ? I also seems mentions to go 1.20 in the CI in multios-unit-tests.yml and in parametric.yml. Can we switch to stable ?

@eliottness Done, thanks! I opted for "oldstable" since that seems to match what we did before (use the oldest supported version). Related, I wanted to use "oldstable" and "stable" everywhere but there were a few places where we need to explicitly provide the Go version. I considered some kind of templating so we could define oldstable & stable in one place but I didn't see an easy way to do that with GitHub Actions. Perhaps defining variables for the repository (like secrets, but not secret)? Can do that as a followup.