build: Bump libevent minimum version up to 2.1.8

Required to support new functionality.

configure.ac

@@ -1526,9 +1526,9 @@ fi
 dnl libevent check
 
 if test "$build_bitcoin_cli$build_bitcoind$bitcoin_enable_qt$use_tests$use_bench" != "nonononono"; then
-  PKG_CHECK_MODULES([EVENT], [libevent >= 2.0.21], [use_libevent=yes], [AC_MSG_ERROR([libevent version 2.0.21 or greater not found.])])
+  PKG_CHECK_MODULES([EVENT], [libevent >= 2.1.8], [use_libevent=yes], [AC_MSG_ERROR([libevent version 2.1.8 or greater not found.])])
   if test "$TARGET_OS" != "windows"; then
-    PKG_CHECK_MODULES([EVENT_PTHREADS], [libevent_pthreads >= 2.0.21],, [AC_MSG_ERROR([libevent_pthreads version 2.0.21 or greater not found.])])
+    PKG_CHECK_MODULES([EVENT_PTHREADS], [libevent_pthreads >= 2.1.8], [], [AC_MSG_ERROR([libevent_pthreads version 2.1.8 or greater not found.])])
   fi
 
   if test "$suppress_external_warnings" != "no"; then

doc/dependencies.md

@@ -18,7 +18,7 @@ You can find installation instructions in the `build-*.md` file for your platfor
 | Dependency | Version used | Minimum required | Runtime |
 | --- | --- | --- | --- |
 | [Boost](https://www.boost.org/users/download/) | 1.77.0 | [1.64.0](https://github.com/bitcoin/bitcoin/pull/22320) | No |
-| [libevent](https://github.com/libevent/libevent/releases) | 2.1.12-stable | [2.0.21](https://github.com/bitcoin/bitcoin/pull/18676) | No |
+| [libevent](https://github.com/libevent/libevent/releases) | 2.1.12-stable | [2.1.8](https://github.com/bitcoin/bitcoin/pull/24681) | No |
 | [glibc](https://www.gnu.org/software/libc/) | N/A | [2.18](https://github.com/bitcoin/bitcoin/pull/23511) | Yes |
 
 ## Optional