Affected: Go: github.com/golang-jwt/jwt/v5, Go: github.com/golang-jwt/jwt/v4, Go: github.com/golang-jwt/jwt

### Summary

Function [`parse.ParseUnverified`](https://github.com/golang-jwt/jwt/blob/c035977d9e11c351f4c05dfeae193923cbab49ee/parser.go#L138-L139) currently splits (via a call to [strings.Split](https://pkg.go.dev/strings#Split)) its argument (which is untrusted data) on periods.

As a result, in the face of a malicious request whose _Authorization_ header consists of `Bearer ` followed by many period characters, a call to that function incurs allocations to the tune of O(n) bytes (where n stands for the length of the function's argument), with a constant factor of about 16. Relevant weakness: [CWE-405: Asymmetric Resource Consumption (Amplification)](https://cwe.mitre.org/data/definitions/405.html)

### Details

See [`parse.ParseUnverified`](https://github.com/golang-jwt/jwt/blob/c035977d9e11c351f4c05dfeae193923cbab49ee/parser.go#L138-L139)

### Impact

Excessive memory allocation