Refactor Heartbeat HTTP communication to be more WAF friendly

[knoobie]

Describe your motivation

Currently the heartbeat used by Vaadin Flow is using text/plain as content type - which is disregarded because it makes it harder for WAFs to guess the correct content. Some WAFs even block it by default or print a warning... alarming the SOC or other teams..

See e.g. https://blog.sicuranext.com/why-text-plain-is-evil-for-web-application-firewall-and-input-validation/

Describe the solution you'd like

• remove content type (there is no body anyway)
• Optional: return 204 (you also don't send stuff back)

Describe alternatives you've considered

• Allow text/plain in WAF
• Ignore problems
• Tell your ops: don't worry.. its normal

Related to #17728

[Artur-]

Related to vaadin/framework#4167