Fix Redwood restart compatibility with legacy XOR page encoding

[tclinkenbeard-oai]

Even with encryptModes=['disabled'], 7.3 Redwood would randomly enable encryption in simulation. #12667 removed the encryption-at-rest feature, but this occasionally causes restarting test failures when encrypted Redwood pages are read upon restart.

This PR adds minimal test-only decryption support to Redwood, allowing these simulation tests to pass.

The issue was detected in CI for #12915:

fdbserver-7.3.63 -r simulation --crash --logsize 1024MB -f src/foundationdb/tests/restarting/from_7.3.0/ConfigureTestRestart-1.toml --buggify off --seed 105248272
fdbserver-45da11c439588d9f9e3e4ed6076d9e359285aa76 -r simulation --crash --logsize 1024MB -f src/foundationdb/tests/restarting/from_7.3.0/ConfigureTestRestart-2.toml --buggify on --seed 105248273

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[foundationdb-ci]

Result of foundationdb-pr on Linux RHEL 9

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Result of foundationdb-pr-cluster-tests on Linux RHEL 9

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