Modernise Go idioms, clean up deps, add Go 1.24 to CI

[Copilot]

Brings the codebase up to current Go standards: replaces pre-1.18 idioms with their modern equivalents, removes unnecessary dependencies, and extends CI coverage to Go 1.24.

CI / build tooling

• Add 1.24.x to the build matrix
• go mod tidy — removes three unused/deprecated indirect deps (golang.org/x/sys, golang.org/x/tools, golang.org/x/tools/cmd/cover)

README

• GoDoc badge: godoc.org → pkg.go.dev

Go idiom updates

• interface{} → any (testingt.go, matcher.go, stubtestingt_test.go, then/eventually.go, matcher_test.go)
• map[interface{}]bool → properly-typed map[V]bool / map[T]bool in is/contains.go — the generic type parameters were already comparable, so this was always valid
• C-style for i := 0; i < len(x); i++ → for _, v := range x throughout (is/contains.go, is/allof.go, is/anyof.go, is/describe.go, has/haseveryelement.go)
• time.Duration(N)*time.Second → N*time.Second
• fmt.Sprintf("%s", x) → direct string assignment (is/allof.go)
• Remove redundant actual.(io.Reader) assertion — actual is already typed as io.Reader (by/eventually.go)

Bug fix

WithinFiveSeconds was passing 10*time.Second as its timeout — corrected to 5*time.Second.

Dead code removal

Unused actualKeys slice in mapActualMatchesExpected (is/contains.go) — built but never read.

[coveralls]

coverage: 3.888% (+0.05%) from 3.834%
when pulling 212b3ab on copilot/add-builds-against-newer-go-versions
into 89a4236 on master.