The fix for CVE-2025-22868 relies on strings.Count, which isn't ideal
because it precludes failing fast when the token contains an unexpected
number of periods. Moreover, Verify still allocates more than necessary.

Eschew strings.Count in favor of strings.Cut. Some benchmark results:

goos: darwin
goarch: amd64
pkg: golang.org/x/oauth2/jws
cpu: Intel(R) Core(TM) i7-6700HQ CPU @ 2.60GHz
                              │      old       │                 new                 │
                              │     sec/op     │   sec/op     vs base                │
Verify/full_of_periods-8        24862.50n ± 1%   57.87n ± 0%  -99.77% (p=0.000 n=20)
Verify/two_trailing_periods-8      3.485m ± 1%   3.445m ± 1%   -1.13% (p=0.003 n=20)
geomean                            294.3µ        14.12µ       -95.20%

                              │     old      │                  new                   │
                              │     B/op     │     B/op      vs base                  │
Verify/full_of_periods-8          16.00 ± 0%     16.00 ± 0%        ~ (p=1.000 n=20) ¹
Verify/two_trailing_periods-8   2.001Mi ± 0%   1.001Mi ± 0%  -49.98% (p=0.000 n=20)
geomean                         5.658Ki        4.002Ki       -29.27%
¹ all samples are equal

                              │     old     │                 new                  │
                              │  allocs/op  │ allocs/op   vs base                  │
Verify/full_of_periods-8         1.000 ± 0%   1.000 ± 0%        ~ (p=1.000 n=20) ¹
Verify/two_trailing_periods-8   12.000 ± 0%   9.000 ± 0%  -25.00% (p=0.000 n=20)
geomean                          3.464        3.000       -13.40%
¹ all samples are equal

Also, remove all remaining calls to strings.Split.

Updates golang/go#71490

Change-Id: Icac3c7a81562161ab6533d892ba19247d6d5b943
GitHub-Last-Rev: 3a82900
GitHub-Pull-Request: #774
Reviewed-on: https://go-review.googlesource.com/c/oauth2/+/655455
Commit-Queue: Neal Patel <nealpatel@google.com>
Reviewed-by: Roland Shoemaker <roland@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Neal Patel <nealpatel@google.com>
Auto-Submit: Neal Patel <nealpatel@google.com>

Change-Id: I369d3bed0e28427b1e1d416952c3766932ba3773
GitHub-Last-Rev: ca611fe
GitHub-Pull-Request: #724
Reviewed-on: https://go-review.googlesource.com/c/oauth2/+/587017
Auto-Submit: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Shin Fan <shinfan@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>