Use fixed 3.0.4 version of @nomiclabs/hardhat-etherscan dependency

Higher version (^3.1.0) reportedly causes problems with etherscan verification
on Mainnet (`TypeError: customChains is not iterable`). See commit bf90cd5.

Author: michalinacienciala

package.json

@@ -35,7 +35,7 @@
     "@keep-network/hardhat-helpers": "^0.6.0-pre.16",
     "@keep-network/prettier-config-keep": "github:keep-network/prettier-config-keep#d6ec02e",
     "@nomiclabs/hardhat-ethers": "^2.0.2",
-    "@nomiclabs/hardhat-etherscan": "^3.1.0",
+    "@nomiclabs/hardhat-etherscan": "3.0.4",
     "@nomiclabs/hardhat-waffle": "^2.0.1",
     "@openzeppelin/hardhat-upgrades": "^1.12.0",
     "@tenderly/hardhat-tenderly": "^1.0.12",

yarn.lock

@@ -1026,21 +1026,18 @@
   resolved "https://registry.yarnpkg.com/@nomiclabs/hardhat-ethers/-/hardhat-ethers-2.0.5.tgz#131b0da1b71680d5a01569f916ae878229d326d3"
   integrity sha512-A2gZAGB6kUvLx+kzM92HKuUF33F1FSe90L0TmkXkT2Hh0OKRpvWZURUSU2nghD2yC4DzfEZ3DftfeHGvZ2JTUw==
 
-"@nomiclabs/hardhat-etherscan@^3.1.0":
-  version "3.1.0"
-  resolved "https://registry.yarnpkg.com/@nomiclabs/hardhat-etherscan/-/hardhat-etherscan-3.1.0.tgz#7137554862b3b1c914f1b1bf110f0529fd2dec53"
-  integrity sha512-JroYgfN1AlYFkQTQ3nRwFi4o8NtZF7K/qFR2dxDUgHbCtIagkUseca9L4E/D2ScUm4XT40+8PbCdqZi+XmHyQA==
+"@nomiclabs/hardhat-etherscan@3.0.4":
+  version "3.0.4"
+  resolved "https://registry.yarnpkg.com/@nomiclabs/hardhat-etherscan/-/hardhat-etherscan-3.0.4.tgz#b12e3e226a5b73c4a66d0e6943f948bd093b2711"
+  integrity sha512-AZPlnyCYp3YObmhtsFo6RWgY/81fQKRF5h42iV22H4jz9MwP+SWeoB99YVPLnxId2fmAYu3VgCNeE9QpApv06g==
   dependencies:
     "@ethersproject/abi" "^5.1.2"
     "@ethersproject/address" "^5.0.2"
     cbor "^5.0.2"
-    chalk "^2.4.2"
     debug "^4.1.1"
     fs-extra "^7.0.1"
-    lodash "^4.17.11"
     semver "^6.3.0"
-    table "^6.8.0"
-    undici "^5.4.0"
+    undici "^4.14.1"
 
 "@nomiclabs/hardhat-waffle@^2.0.1":
   version "2.0.2"
@@ -9405,7 +9402,7 @@ table@^5.2.3:
     slice-ansi "^2.1.0"
     string-width "^3.0.0"
 
-table@^6.0.9, table@^6.8.0:
+table@^6.0.9:
   version "6.8.0"
   resolved "https://registry.yarnpkg.com/table/-/table-6.8.0.tgz#87e28f14fa4321c3377ba286f07b79b281a3b3ca"
   integrity sha512-s/fitrbVeEyHKFa7mFdkuQMWlH1Wgw/yEXMt5xACT4ZpzWFluehAxRtUUQKPuWhaLAWhFcVx6w3oC8VKaUfPGA==
@@ -9824,6 +9821,11 @@ underscore@1.9.1:
   resolved "https://registry.yarnpkg.com/underscore/-/underscore-1.9.1.tgz#06dce34a0e68a7babc29b365b8e74b8925203961"
   integrity sha512-5/4etnCkd9c8gwgowi5/om/mYO5ajCaOgdzj/oW+0eQV9WxKBDZw5+ycmKmeaTXjInS/W0BzpGLo2xR2aBwZdg==
 
+undici@^4.14.1:
+  version "4.16.0"
+  resolved "https://registry.yarnpkg.com/undici/-/undici-4.16.0.tgz#469bb87b3b918818d3d7843d91a1d08da357d5ff"
+  integrity sha512-tkZSECUYi+/T1i4u+4+lwZmQgLXd4BLGlrc7KZPcLIW7Jpq99+Xpc30ONv7nS6F5UNOxp/HBZSSL9MafUrvJbw==
+
 undici@^5.4.0:
   version "5.9.1"
   resolved "https://registry.yarnpkg.com/undici/-/undici-5.9.1.tgz#fc9fd85dd488f965f153314a63d9426a11f3360b"