splice: Take splice out of experimental mode!

[ddustin]

Turn on splicing by default and remove the experimental splicing feature!

[rustyrussell]

It's good, but a bit overzealous. We need to silently accept it and run a full deprecation period. This avoids breaking people's configs when they upgrade (and by the time we do remove it, in 12 months, they don't have to worry about downgrading).

See this example:

static char *opt_set_hsm_password(struct lightningd *ld)
{
	int is_encrypted;

	/* Show deprecation warning */
	if (!opt_deprecated_ok(ld, "encrypted_hsm",
			      "Use --hsm-passphrase instead",
			      "v25.12", "v26.12"))
		return "--encrypted-hsm was removed, use --hsm-passphrase instead";

Then update doc/developers-guide/deprecations.md, and Changelog-Deprecated it.