docs: Remove obsolete log attribute prefix limitations

[chrkl]

Description

PR #590 changed the Fluent Bit configuration to use one Kubernetes filter per LogPipeline. This also avoids filters for JSON manipulation that used the __kyma__ attribute prefix.

Changes proposed in this pull request (what was done and why):

• Remove obsolete log attribute prefix limitations

Changes refer to particular issues, PRs or documents:

• Log agent based on container runtime as buffer instead of own filesystem buffer #479
• feat: Use separate tail plugin per LogPipeline #590

Traceability

• The PR is linked to a GitHub issue.
• New features have a milestone set.
• New features have defined acceptance criteria in a corresponding GitHub Issue, and all criteria are satisfied with this PR.
• The corresponding GitHub issue has a respective area and kind label.
• The follow-up issues (if any) are linked in the Related Issues section.
• Adjusted the documentation if the change is user-facing.
• The feature is unit-tested
• The feature is e2e-tested