go.mod: bump minimum and language to 1.21 (#949)

* go.mod: bump minimum and language to 1.21

CI only tests Go 1.21 and 1.22, and older versions of Go are no longer
getting any bug or security fixes, so advertise that we only support
Go 1.21 or later via go.mod.

While here, ensure the module is tidy and resolve deprecation warnings,
and remove now-unnecessary Go version build tags.

* replace sort.Slice with slices.SortFunc

The latter is more efficient, and allocates less, since sort.Slice
needs to go through sort.Interface which causes allocations.

    goos: linux
    goarch: amd64
    pkg: github.com/pelletier/go-toml/v2/benchmark
    cpu: AMD Ryzen 7 PRO 5850U with Radeon Graphics
                              │     old     │                new                 │
                              │   sec/op    │   sec/op     vs base               │
    Marshal/HugoFrontMatter-8   7.612µ ± 1%   6.730µ ± 1%  -11.59% (p=0.002 n=6)

                              │     old      │                 new                 │
                              │     B/s      │     B/s       vs base               │
    Marshal/HugoFrontMatter-8   65.52Mi ± 1%   74.11Mi ± 1%  +13.11% (p=0.002 n=6)

                              │     old      │                new                 │
                              │     B/op     │     B/op      vs base              │
    Marshal/HugoFrontMatter-8   5.672Ki ± 0%   5.266Ki ± 0%  -7.16% (p=0.002 n=6)

                              │    old     │                new                │
                              │ allocs/op  │ allocs/op   vs base               │
    Marshal/HugoFrontMatter-8   85.00 ± 0%   73.00 ± 0%  -14.12% (p=0.002 n=6)

fuzz_test.go

@@ -1,6 +1,3 @@
-//go:build go1.18 || go1.19 || go1.20 || go1.21 || go1.22
-// +build go1.18 go1.19 go1.20 go1.21 go1.22
-
 package toml_test
 
 import (