build: upgrade to golang 1.21.3

[holiman]

Supersedes #28268

---

Upgrades to 1.21.2 1.21.3.

This PR also includes a tool to sanity-check the checksums in our checksum list. I expanded the semantics of the checksum txt file, so that a line starting with # https sets a new base-url, and we can download and verify each hash/file combo under the given base url.

Execute with go run ./build/ci.go sanitycheck.

Also, if we decide this is a nice idea, then we can make the fixtures-download use this scheme, and we don't have to have a fixtures-version in the main ci.go -- this part:

func downloadSpecTestFixtures(csdb *build.ChecksumDB, cachedir string) string {
	ext := ".tar.gz"
	base := "fixtures" // TODO(MariusVanDerWijden) rename once the version becomes part of the filename
	url := fmt.Sprintf("https://github.com/ethereum/execution-spec-tests/releases/download/v%s/%s%s", executionSpecTestsVersion, base, ext)

We could instead just infer it from the checksumdb.

[holiman]

Food for though, for triage;

We could structure the checksums like so:

# resource:fixtures
# https://github.com/ethereum/execution-spec-tests/releases
# https://github.com/ethereum/execution-spec-tests/releases/download/v1.0.2/
24bac679f3a2d8240d8e08e7f6a70b70c2dabf673317d924cf1d1887b9fe1f81  fixtures.tar.gz

# resource:golang
# https://go.dev/dl/
186f2b6f8c8b704e696821b09ab2041a5c1ee13dcbc3156a13adcf75931ee488  go1.21.3.src.tar.gz
27014fc69e301d7588a169ca239b3cc609f0aa1abf38528bf0d20d3b259211eb  go1.21.3.darwin-amd64.tar.gz

Where resource: would be a keyword, so that we would not longer have to define dlgoVersion and executionSpecTestsVersion; updating either of them would just mean that we update the checksums.txt-file.

[fjl]

# version:golang 1.21.3

type ChecksumDB struct{
    versions map[string]string    // "golang" -> "1.21.3"
}

[karalabe]

Closing this in favor of the followup pr.