fix(gateway): be more lenient with prefix paths trailing slashes

[michaelbeaumont]

This check breaks existing resources so we should remove it. No logic is changed here because the validation was more of a guide rail to prevent misunderstandings around how exactly prefixes are matched.

The trailing slash is essentially ignored and so the new golden file should be identical to the existing pkg/plugins/runtime/gateway/testdata/http/rewrite-prefix-gateway-route.yaml golden file.

This adds e2e tests for trailing slashes too.

Checklist prior to review

• Link to docs PR or issue --
• Link to UI issue or PR --
• Is the issue worked on linked? --
• The PR does not hardcode values that might break projects that depend on kuma (e.g. "kumahq" as a image registry) --
• The PR will work for both Linux and Windows, system specific functions like syscall.Mkfifo have equivalent implementation on the other OS --
• Unit Tests --
• E2E Tests --
• Manual Universal Tests -- none
• Manual Kubernetes Tests --
• Do you need to update UPGRADE.md? --
• Does it need to be backported according to the backporting policy? --
• Do you need to explicitly set a > Changelog: entry here or add a ci/ label to run fewer/more tests?