[APM] remove transaction.page field

[axw]

In 7.9.0, APM Server started duplicating the non-standard RUM-specific transaction.page.* fields under the ECS url.* and http.* fields. I would like to remove the old transaction.page.* fields. I think we can mostly leave the UI code untouched so it can continue to visualise old RUM data, except for this snippet:

kibana/x-pack/plugins/apm/public/components/shared/Summary/TransactionSummary.tsx

Lines 27 to 30 in a923d9e

	const isRumAgent = isRumAgentName(transaction.agent.name);
	const url = isRumAgent
	? transaction.transaction.page?.url
	: transaction.url?.full;

If we replace this with something like the following, then I think we we're good:

const url = transaction.url?.full || transaction.transaction?.page?.url;

[elasticmachine]

Pinging @elastic/apm-ui (Team:apm)

[sorenlouv]

Sgtm 👍