Fix mentions of buidler (#2548)

(cherry picked from commit 232c355b3ab6c14b8eb6c8127376995dcba8bf3e)

.gitignore

@@ -54,6 +54,6 @@ allFiredEvents
 .coverage_cache
 .coverage_contracts
 
-# buidler
+# hardhat
 cache
 artifacts

contracts/proxy/README.adoc

@@ -13,7 +13,7 @@ An alternative upgradeability mechanism is provided in <<Beacon>>. This pattern,
 
 The {Clones} library provides a way to deploy minimal non-upgradeable proxies for cheap. This can be useful for applications that require deploying many instances of the same contract (for example one per user, or one per task). These instances are designed to be both cheap to deploy, and cheap to call. The drawback being that they are not upgradeable.
 
-CAUTION: Using upgradeable proxies correctly and securely is a difficult task that requires deep knowledge of the proxy pattern, Solidity, and the EVM. Unless you want a lot of low level control, we recommend using the xref:upgrades-plugins::index.adoc[OpenZeppelin Upgrades Plugins] for Truffle and Buidler.
+CAUTION: Using upgradeable proxies correctly and securely is a difficult task that requires deep knowledge of the proxy pattern, Solidity, and the EVM. Unless you want a lot of low level control, we recommend using the xref:upgrades-plugins::index.adoc[OpenZeppelin Upgrades Plugins] for Truffle and Hardhat.
 
 == Core