searcher: replace usage of gitserver.DiffSymbols with new gitserver.ChangedFiles client method

[ggilmore]

Part of https://github.com/sourcegraph/sourcegraph/issues/60654

This PR replaces hybrid's search use of the gitserver.DiffSymbols endpoint with the new gitserver.ChangedFiles gRPC endpoint introduced in https://github.com/sourcegraph/sourcegraph/pull/62354.

Test plan

Existing CI pipeline

[ggilmore]

• #62363
• #62360
• #62359
• #62358 👈
• #62355
• #62354
• #62262
• #62252
• #62217
• #62216
• #62212
• #62302
• #62173
• #62169
• #62165
• #62158
• main

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Graphite Automations

"Post a GIF when PR approved" took an action on this PR • (05/02/24)

1 gif was posted to this PR based on Geoffrey Gilmore's automation.

"Auto-assign PRs to author" took an action on this PR • (05/06/24)

3 assignees were added to this PR based on Geoffrey Gilmore's automation.

[keegancsmith]

meta: github does a really bad job with all these graphite updates. The actual review comments all get hidden away with the inconsiquential updates on force-pushes.

[ggilmore]

Merge activity

• May 7, 2:10 PM EDT: Graphite rebased this pull request as part of a merge.
• May 7, 2:20 PM EDT: @ggilmore merged this pull request with Graphite.