Bump changelog and VERSION for v4.0.0-beta.2

[DrJosh9000]

v4.0.0-beta.2 (2026-05-06)

Changed since beta 1

• Make pipeline secret redaction default behaviour #3897 (@moskyb, @DrJosh9000)
• Rip out opentracing tracing backend (take 2) #3896 (@moskyb, @DrJosh9000)
• Replace cancel-grace-period and signal-grace-period-seconds with cancel-signal-timeout and cancel-cleanup-timeout #3868 (@moskyb)
• fix: Reverse ordering for post- hooks #3853 (@DrJosh9000)

Changed since v3.125.0

• build(deps): bump github.com/buildkite/go-buildkite/v4 from 4.19.0 to 4.21.0 #3860 (@dependabot[bot])
• build(deps): bump the cloud-providers group across 1 directory with 5 updates #3879 (@dependabot[bot])
• build(deps): bump github.com/klauspost/compress from 1.18.5 to 1.18.6 #3885 (@dependabot[bot])
• build(deps): bump the container-images group across 1 directory with 3 updates #3888 (@dependabot[bot])
• build(deps): bump the container-images group across 2 directories with 1 update #3895 (@dependabot[bot])
• build(deps): bump the container-images group across 4 directories with 1 update #3886 (@dependabot[bot])
• build(deps): bump the container-images group across 2 directories with 1 update #3890 (@dependabot[bot])
• build(deps): bump the container-images group across 3 directories with 1 update #3891 (@dependabot[bot])
• docs: mention BUILDKITE_S3_PROFILE in artifact upload help #3869 (@mastermanas805)