Drop unsampled transactions when connected to APM Server 8.0+

[felixbarny]

As of 8.0, there's no option to keep unsampled transactions for backend agents (see also elastic/apm-server#6514).

Therefore, agents should not send unsampled transactions when they're connected to an APM Server that's version 8.0 or higher.

Spec issue

• [META 545] Agent Spec: Drop unsampled transactions #552

Agent issues

• [META 545] Drop unsampled transactions when connected to APM Server 8.0+ apm-agent-java#2276
• [META 545] Drop unsampled transactions when connected to APM Server 8.0+ apm-agent-dotnet#1573
• [META 545] Drop unsampled transactions when connected to APM Server 8.0+ apm-agent-go#1153
• [META 545] Drop unsampled transactions when connected to APM Server 8.0+ apm-agent-php#576
• [META 545] Drop unsampled transactions when connected to APM Server 8.0+ apm-agent-ruby#1194
• [META 545] Drop unsampled transactions when connected to APM Server 8.0+ apm-agent-nodejs#2455
• [META 545] Drop unsampled transactions when connected to APM Server 8.0+ apm-agent-python#1408