[chore][processor/redactionprocessor] Updating to latest semconv version

[anarwal]

Description

1. internal/url/spanname.go — Removed semconv125 import, renamed semconv138 to conventions. Replaced old HTTP constants (HTTPSchemeKey, HTTPTargetKey, HTTPMethodKey, HTTPURLKey) with string literals. The code already checks both old and new HTTP keys for dual-support.
2. internal/db/spanname.go — Removed semconv128 import, renamed semconv138 to conventions. Replaced semconv128.DBSystemKey with "db.system". The code already checks db.system.name (new) first, falling back to db.system (old).

Link to tracking issue

Fixes processor/redactionprocessor from #22095

Testing

Tested by go mod tidy && go build ./... && go test ./... from processor/redactionprocessor

Authorship

• I, a human, wrote this pull request description myself.