wallet, refactor: rename file from migrate to legacybdb #34910

The wallet/migrate.h|cpp file contain all the code related to the Berkeley DB that was used in the legacy wallets. Although it's true that this code is used only within the context of wallet migration but the name of the file is not exact.

This patch renames the file to legacybdb so that the intent is clearer in the call sites. I felt this rename might be necessary so that the name doesn't conflict with the upcoming wallet/migration.h|cpp files in #34909.

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Code Coverage & Benchmarks

For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/34910.

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Conflicts

Reviewers, this pull request conflicts with the following ones:

• #34909 (wallet, refactor: modularise wallet by extracting out legacy wallet migration by rkrux)

If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

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Rebased over master to incorporate changes from #35227.

Do you think it makes sense to rename it to migrate_legacybdb(.h|.cpp) instead, so we preserve the migrate wording?

In #34909, migration.cpp|h files are added that covers all the migration workflow. I want to avoid having the migrate naming in these files because all the code here is within the context of legacy BDB, with the exception of couple comments that mention the migration workflow. We have sqlite.cpp|h files in the wallet directory, it seemed consistent to me to have corresponding entries for BDB but with the legacy prefix appended so that it's clear that all this code is not used now.