do not overwrite vehicle maxspeed with higher value, fixes #6979

[woodpeck]

Issue

Fix the overwriting of a lower vehicle-based maxspeed by a higher signposted (edge-based) maxspeed

Tasklist

• [x] CHANGELOG.md entry (How to write a changelog entry)
• [ ] update relevant Wiki pages
• [ ] add tests (see testing documentation)
• [ ] review
• [ ] adjust for comments
• [ ] cherry pick to release branch

Requirements / Relations

Link any requirements here. Other pull requests this PR is based on?