T-20356 Make slog-betterstack the official Better Stack Go client

[PetrHeinz]

Makes github.com/BetterStackHQ/slog-betterstack the official Better Stack client for Go's log/slog. The code is samber/slog-betterstack as of its main branch (v1.4.4 plus dependency bumps) with its full git history, taken as-is: this PR changes the module path, the project files and the repository setup so that the library looks and releases like our other clients. The behaviour fixes (batching, flush on close, retries, surfacing delivery errors) come in a follow-up PR.

• Module path github.com/BetterStackHQ/slog-betterstack, package name slogbetterstack and the API unchanged, so upgrading is an import path change. logger.name in every record and the User-Agent now say BetterStackHQ/slog-betterstack. logger.version comes from version.go, which the Release workflow bumps. It says 1.4.4, and that is the version this lands as, the same code at its new path: tag the merge commit v1.4.4 by hand, push the tag and dispatch retry from main, which creates the GitHub release and primes the Go module proxy. The fixes PR then releases as 1.5.0.
• README in the house layout: license, latest version and CI badges (the version badge links to pkg.go.dev, Go's package index), the docs link and credits to Samuel Berthe, nothing else. Usage documentation lives at betterstack.com/docs/logs/go. The sponsor banners and the catalogue of his other handlers are gone. LICENSE keeps his MIT copyright line and adds Better Stack's.
• main.yml builds, vets and runs the tests with the race detector on Go 1.21 through 1.25 and stable, on every push and PR, weekly and on demand, plus gofmt, go mod tidy and golangci-lint checks. release.yml is the Ruby and Python clients' workflow ported to a Go module: patch, minor, major and retry runs plus dry runs, the version bump in version.go, the version commit and annotated tag pushed by the workflow, a GitHub release with generated notes, and a go list -m through proxy.golang.org so the version is cached and indexed immediately. A major release also moves the module path to /vN in go.mod, the example import and the README's pkg.go.dev links in the same version commit, as Go's semantic import versioning requires. Every push touching the file is a dry run, so this PR's checks include one. samber's test, lint and automerge workflows and FUNDING.yml are removed; Dependabot stays monthly.
• handler_test.go pins what applications rely on: the record keys (dt, level, message, logger.name, logger.version, extra), the error and runtime shapes, the request headers, the defaults and every option. The tests look only at what reaches the server, so the follow-up can change the transport underneath them. goleak's TestMain finally has tests to guard. Statement coverage is 100%.
• example/ became example-project/ like in the other clients and reads the token and ingesting host from the environment.
• handler.go is gofmt-ed. No other code change.

Verified locally: go test -race green on Go 1.25, golangci-lint clean, actionlint clean. The example sent three records to a test source on the clanker team and they arrived with the new logger.name. The Release bump step was rehearsed on a clone: 1.4.4 → 1.5.0 for minor, 2.0.0 with the module moved to /v2 and a passing build for major, retry refuses without a tag, a tagged HEAD refuses patch, and retry on a tagged HEAD checks the tag out and reads the module path from it.

Repository setting this relies on: main is unprotected today and the Release workflow's version push needs it to stay that way, or the GitHub Actions app must be allowed to bypass the pull request requirement as in the Ruby repos.

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