fix(git): recognize uppercase URL schemes when parsing remotes

What does this MR do and why?

git.ParseURL misparses a remote URL whose scheme is uppercase. The protocol checks only match lowercase prefixes:

func isSupportedProtocol(u string) bool {
	return strings.HasPrefix(u, "ssh:") || ... || strings.HasPrefix(u, "https:")
}

HasPrefix("HTTPS://...", "https:") is false, so ParseURL falls through to its scp-style SSH transform:

HTTPS://example.com/owner/repo.git  =>  scheme="ssh" host="HTTPS" path="///example.com/owner/repo.git"

URL schemes are case-insensitive (RFC 3986), and net/url.Parse already lowercases the scheme; only glab's pre-check was case-sensitive. The checks now compare in lower case, so an uppercase scheme parses the same as the lowercase form.

Test coverage

An uppercase HTTPS scheme case added to TestParseURL, expecting scheme="https", host="example.com". It fails on the old code (host="HTTPS"). The existing scheme cases are unchanged.

go test ./internal/git/ is green.

Related issues

Closes #8435 (closed)