time: Parse of RFC3339 is incorrect for timezone field

RFC 3339, section 5.6 specifies that the `time-numoffset` field must be either a minus or plus sign followed by 2 digits a colon and 2 digits.

However, [`Parse` permits the integer fields having plus and minus signs](https://go.dev/play/p/j1SONY_Xpa6):
```go
Parse(time.RFC3339, "0000-01-01T00:00:00+-0:+0")
```
reports nil rather than an error.