Support Go 1.24-1.27 and modernize deprecated APIs

[const-cloudinary]

Summary

Moves the CI matrix and README support table to Go 1.24–1.27, and fixes the things that stood in the way of those versions actually passing.

Two of the fixes were only discoverable in CI, since the test suite needs real credentials against a live test cloud:

• gotestsum@v1.11.0 does not build on Go 1.25+. It depends on golang.org/x/tools@v0.11.0, which fails to compile with invalid array length -delta * delta in internal/tokeninternal. This aborted the job before a single test ran, so the new matrix rows would have been permanently red regardless of the SDK code. Bumped to v1.13.0 (pulls x/tools@v0.36.0).
• Unsigned underflow in watchMemoryAllocation. read - initialRead was computed on uint64 without checking that the live heap had grown. runtime.MemStats.Alloc is the live heap, so a GC cycle during the upload can drop it below the baseline; the subtraction then wraps to ~2^64 and trips the allocation limit. This failed TestUploader_UploadLargeFile with allocated 18446744073709509944 bytes. The bug is pre-existing — Go 1.26's GC timing just makes it reproducible.

Changes

Version support

• CI matrix: 1.20–1.24 → 1.24, 1.25, 1.26, 1.27
• README support table collapsed to Go 1.13 - 1.19 / Go 1.20 - 1.23 / Go 1.24 - 1.27
• gotestsum v1.11.0 → v1.13.0

Deprecated APIs

• ioutil.ReadAll / ioutil.TempFile → io.ReadAll / os.CreateTemp
• strings.Title → explicit first-rune upper
• rand.Seed → local rand.New source

Correctness

• Upload discarded the error from HandleRawResponse and returned nil
• Typed context key for api_version instead of a bare string (staticcheck SA1029)
• watchMemoryAllocation: only count heap growth; make proceed/maxAlloc atomic, as they were written by the returned closure and read by the sampling goroutine unsynchronized
• TestUploader_UploadWithResponsiveBreakpoints dereferenced a nil response after t.Error → t.Fatal
• getAutoVideoDetailsTestCases was defined but never registered in the acceptance suite
• Dropped duplicated omitempty on UpdateAssetParams.Tags
• Raw string literal for the base64 detection regex
• Removed unused test constants and an unused assignment

Test plan

CI green on all four versions — 279 tests, 3 skipped, 0 failures on Go 1.24, 1.25, 1.26, and 1.27. The 1.27 job confirms go version go1.27.0 linux/amd64, so it is not silently falling back to an older toolchain. The 3 skips are the pre-existing dynamic_folders / visual_search account-feature gates.

Go 1.27 was additionally verified locally (build, vet, and a gotestsum v1.13.0 install) before being added to the matrix.

Note

go.mod still declares go 1.20, left deliberately unchanged. Dropping the older CI rows doesn't by itself end support for those versions — the directive is what enforces the floor. Raising the minimum is a separate decision with downstream impact.

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