Remove minimum master nodes bootstrap check

This commit removes the minimum master nodes bootstrap check. The
motivation for this check was to raise awareness of the minimum master
nodes setting but this check gives a false sense of security because
it's too easy to set the setting to one when first standing up a cluster
and never update it when adding master-eligible nodes, or have it out of
sync on various nodes and still pass this check. Since this check does
not have the security that other bootstrap checks provide, it should be
removed in favor of a stronger guarantee in the future. We do log a
warning if an election occurs with minimum master nodes less than a
quorum of master-eligible nodes that participated in an election and
this is the best that we can do right now.

Relates #20082

core/src/main/java/org/elasticsearch/bootstrap/BootstrapCheck.java

@@ -166,7 +166,6 @@ final class BootstrapCheck {
         if (Constants.LINUX || Constants.MAC_OS_X) {
             checks.add(new MaxSizeVirtualMemoryCheck());
         }
-        checks.add(new MinMasterNodesCheck(ElectMasterService.DISCOVERY_ZEN_MINIMUM_MASTER_NODES_SETTING.exists(settings)));
         if (Constants.LINUX) {
             checks.add(new MaxMapCountCheck());
         }
@@ -330,32 +329,6 @@ final class BootstrapCheck {
 
     }
 
-    static class MinMasterNodesCheck implements Check {
-
-        final boolean minMasterNodesIsSet;
-
-        MinMasterNodesCheck(boolean minMasterNodesIsSet) {
-            this.minMasterNodesIsSet = minMasterNodesIsSet;
-        }
-
-        @Override
-        public boolean check() {
-            return minMasterNodesIsSet == false;
-        }
-
-        @Override
-        public String errorMessage() {
-            return "please set [" + ElectMasterService.DISCOVERY_ZEN_MINIMUM_MASTER_NODES_SETTING.getKey() +
-                "] to a majority of the number of master eligible nodes in your cluster";
-        }
-
-        @Override
-        public final boolean isSystemCheck() {
-            return false;
-        }
-
-    }
-
     static class MaxNumberOfThreadsCheck implements Check {
 
         private final long maxNumberOfThreadsThreshold = 1 << 11;

core/src/test/java/org/elasticsearch/bootstrap/BootstrapCheckTests.java

@@ -385,15 +385,6 @@ public class BootstrapCheckTests extends ESTestCase {
         BootstrapCheck.check(true, false, Collections.singletonList(check), "testMaxMapCountCheck");
     }
 
-    public void testMinMasterNodes() {
-        boolean isSet = randomBoolean();
-        BootstrapCheck.Check check = new BootstrapCheck.MinMasterNodesCheck(isSet);
-        assertThat(check.check(), not(equalTo(isSet)));
-        List<BootstrapCheck.Check> defaultChecks = BootstrapCheck.checks(Settings.EMPTY);
-
-        expectThrows(RuntimeException.class, () -> BootstrapCheck.check(true, false, defaultChecks, "testMinMasterNodes"));
-    }
-
     public void testClientJvmCheck() {
         final AtomicReference<String> vmName = new AtomicReference<>("Java HotSpot(TM) 32-Bit Client VM");
         final BootstrapCheck.Check check = new BootstrapCheck.ClientJvmCheck() {