Bump collector version to 0.104.0; adjust environment variable expansion

[matej-g]

Description

• Bumps collector to 0.104.0
• Adjusts environment variables expansion as instructed in https://github.com/open-telemetry/opentelemetry-collector-contrib/releases/tag/v0.104.0 and update our READMEs to make it clear to customers
• Add feature gates to still accept the old way of setting env vars (will be removed in one of the next releases)

How Has This Been Tested?

Locally

Checklist:

• I have updated the relevant Helm chart(s) version(s)
• I have updated the relevant component changelog(s)
• This change does not affect any particular component (e.g. it's CI or docs change)