NarInfoDiskCache: Rename cacheExists -> upToDateCacheExists

This is slightly more accurate considering that an outdated record
may exist in the persistent cache. Possibly-outdated records are
quite relevant as they may be foreign keys to more recent information
that we want to keep, but we will not return them here.

(cherry picked from commit 29f0b196f4)

src/libstore/nar-info-disk-cache.cc

@@ -200,7 +200,7 @@ public:
         });
     }
 
-    std::optional<CacheInfo> cacheExists(const std::string & uri) override
+    std::optional<CacheInfo> upToDateCacheExists(const std::string & uri) override
     {
         return retrySQLite<std::optional<CacheInfo>>([&]() -> std::optional<CacheInfo> {
             auto state(_state.lock());