ci: install Semgrep without hash pinning

The hash-pinned requirements-semgrep.txt kept breaking: Semgrep pins its
own dependencies to exact or compatible-release versions, so Dependabot
bumps of any of them (importlib-metadata, exceptiongroup, jsonschema,
peewee, wcmatch, wrapt, ...) made the job fail with ResolutionImpossible.
Install the latest Semgrep with pip instead, remove the lock file and the
Dependabot ignore list that tried to work around it.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

.github/dependabot.yml

@@ -60,11 +60,3 @@ updates:
       interval: daily
     cooldown:
       default-days: 7
-    ignore:
-      # semgrep pins these packages to exact or compatible-release versions (see its METADATA);
-      # bumping them on their own makes the Semgrep job fail with ResolutionImpossible, so
-      # they only move together with a semgrep release
-      - dependency-name: mcp
-      - dependency-name: boltons
-      - dependency-name: "opentelemetry-*"
-      - dependency-name: importlib-metadata