ParseAcceptLanguage quadratic-time DoS via Locale middleware on unauthenticated requests

Affected: Go: code.gitea.io/gitea

### Summary

The Locale middleware that runs in front of every unauthenticated request calls `golang.org/x/text/language.ParseAcceptLanguage` on the raw `Accept-Language` header without imposing a size or shape filter. The underlying parser has quadratic-time behaviour on long lists of malformed language tags. The CVE-2022-32149 guard that golang.org/x/text added in v0.3.8 caps the number of `-` characters in the input at 1000, but it does not cap `_` characters even though the parser's internal scanner aliases `_` to `-` before parsing. A single unauthenticated GET request with an `Accept-Language` header built out of `_` separators burns ~2 seconds of server CPU on the host running Gitea; ten concurrent attackers saturate a ten-core box for the duration of the attack while consuming ~1 MiB of upstream bandwidth per request.

### Affected versions

`code.gitea.io/gitea` 1.22.6 and (per code inspection of `main`) all earlier and later 1.22.x / 1.23.x / 1.24.x / 1.25.x / 1.26.x versions that do not impose their own size limit on the `Accept-Language` header before calling `ParseAcceptLanguage`. Verified on:

- the official `gitea/gitea:1.22.6` docker image (E2E below) - `main` at commit `6f4027a6be28c876c0abaf37cc939658645b78a3` by reading `modules/web/middleware/locale.go` (the call site at line 38 is unchanged on `main`)

### Privilege required

Unauthenticated. The Locale middleware runs for every HTTP request including the landing page and the sign-in page.

### Vulne…