[exporter/datadog] Remove automatic env variable detection

[mx-psi]

Description:

• Remove automatic environment variable detection on the Datadog exporter
• Add a warning if configuration would be resolved different prior to this change.
• Remove TagsConfig.GetHostTags method

Link to tracking Issue: fixes #8396, updates #8373

Testing:

• Test that warning is logged when using the automatic environment variable detection
• Test that this warning is not logged when using the environment variable explicitly

Documentation: Remove references to automatic environment variable detection from docs

[mx-psi]

This should not be merged until after the v0.49.0 release

[mx-psi]

Per #8372 (comment) I am closing this until we remove all deprecated settings in one go.