build: remove the Dependabot ignore for which

[juliasilge]

This PR removes the Dependabot ignore rule for which 7 and later from .github/dependabot.yml.

#1198 added this rule, because which 7 needs Node >=22.22, and the extension host in VS Code 1.101 uses an older Node.

#1199 removed which as a direct dependency. A local findOnPath helper in apps/vscode/src/core/python.ts now does the same work. As a result, the ignore rule has no function.

Other packages still bring in which 2 as a transitive dependency. This change does not affect it.

#1201 (the Dependabot bump of which from 3.0.1 to 6.0.1) is also obsolete after #1199, but thankfully Dependabot closed it.

[posit-snyk-bot]

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