symbols: replace usage of gitserver client's DiffSymbols method with new ChangedFiles method

[ggilmore]

Part of #60654

This PR builds on the new ChangedFiles method introduced in #62354, and changes the symbols service to use it instead of the old custom diff symbols method.

Test plan

Existing CI pipeline

[ggilmore]

• searcher: remove unused diff.ParseGitDiffNameStatus package #62363
• gitserver: grpc: remove now unused DiffSymbols func #62360
• worker: embeddings: replace usage of old gitserver.DiffSymbols client method with new gitserver.ChangedFiles method #62359
• searcher: replace usage of gitserver.DiffSymbols with new gitserver.ChangedFiles client method #62358
• symbols: replace usage of gitserver client's DiffSymbols method with new ChangedFiles method #62355 👈
• gitserver: grpc: add new ChangedFiles method to gitserver client #62354
• gitserver: grpc: create changed files RPC implementation for gRPC server #62262
• gitserver: grpc: port DiffSymbols from client as new GitBackend.ChangedFiles() func #62252
• gitserver: grpc: swap client implementation of GetAheadBehind from Exec to bespoke gRPC endpoint #62217
• gitserver: grpc: create server implementation for GetBehindAhead #62216
• gitserver: grpc: port GetBehindAhead from client to gitcli backend #62212
• codeinsights: remove now useless empty repo string error check #62302
• gitserver: grpc: switch clientImplementor to use new dedicated gRPC method for FirstEverCommit #62173
• gitserver: grpc: create FirstEverCommit RPC method and server implementation #62169
• gitserver: port FirstEverCommit functionality from client to git cli backend #62165
• gitserver: grpc: create exhaustive logger for repository service #62158
• main

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Merge activity

• May 7, 1:59 PM EDT: Graphite rebased this pull request after merging its parent, because this pull request is set to merge when ready.
• May 7, 2:08 PM EDT: @ggilmore merged this pull request with Graphite.