Lower log message level for nailgun to reduce verbosity (#20946)

Nailgun starting log message makes Pants output very verbose and could
actually feel somewhat annoying.

docs/notes/2.22.x.md

@@ -56,6 +56,10 @@ deploy jars, in addition to those specified on a per-jar basis in the `deploy_ja
 This option's default value excludes signature files from constituent jars, which are known to cause the deploy jar
 to fail to execute (since naturally it doesn't match those signatures).
 
+Also reduced verbosity of log messages for the internal Nailgun server instances used for running or compiling code
+replacing it by a single message informing about the size of the Nailgun pool. Previous log messages are still
+emitted at debug log level.
+
 The internal code for exporting JVM tools was refactored.
 
 ##### Scala

src/python/pants/core/goals/check_test.py

@@ -157,7 +157,7 @@ def run_typecheck_rule(
 ) -> Tuple[int, str]:
     union_membership = UnionMembership({CheckRequest: request_types})
     check_subsystem = create_subsystem(CheckSubsystem, only=only or [])
-    with mock_console(create_options_bootstrapper()) as (console, stdio_reader):
+    with mock_console(create_options_bootstrapper(["-lwarn"])) as (console, stdio_reader):
         rule_runner = RuleRunner()
         result: Check = run_rule_with_mocks(
             check,

src/rust/engine/process_execution/pe_nailgun/src/nailgun_pool.rs

@@ -67,6 +67,7 @@ pub struct NailgunPool {
 
 impl NailgunPool {
     pub fn new(workdir_base: PathBuf, size: usize, store: Store, executor: Executor) -> Self {
+        info!("Initializing Nailgun pool for {} processes...", size);
         NailgunPool {
             workdir_base,
             size,
@@ -285,9 +286,7 @@ fn spawn_and_read_port(
     workdir: PathBuf,
 ) -> Result<(std::process::Child, Port), String> {
     let cmd = process.argv[0].clone();
-    // TODO: This is an expensive operation, and thus we info! it.
-    //       If it becomes annoying, we can downgrade the logging to just debug!
-    info!(
+    debug!(
         "Starting new nailgun server with cmd: {:?}, args {:?}, in cwd {}",
         cmd,
         &process.argv[1..],