Rename HTTP headers without the deprecated X- prefix

[kevin-dp]

The sync service uses custom headers starting with x-, however, this convention has been deprecated since 2012 and is actually advised against doing that (cf. https://datatracker.ietf.org/doc/html/rfc6648#page-4).

3. Recommendations for Creators of New Parameters

Creators of new parameters to be used in the context of application
protocols:

1. SHOULD assume that all parameters they create might become standardized, public, commonly deployed, or usable across multiple implementations.
2. SHOULD employ meaningful parameter names that they have reason to believe are currently unused.
3. SHOULD NOT prefix their parameter names with "X-" or similar constructs.

Note: If the relevant parameter name space has conventions about associating parameter names with those who create them, a parameter name could incorporate the organization's name or primary domain name (see Appendix B for examples).

We already follow their advise of incorporating our organisation name, so we should rename headers like x-electric-shape-id to electric-shape-id.