a tiny update: it seemed to me from the log that jekyll-admin required some deprecated Gem, but even after removing it, the error seems to be the same. The new log:
bundler: failed to load command: jekyll (/usr/local/lib/ruby/gems/3.0.0/bin/jekyll)
/usr/local/lib/ruby/gems/3.0.0/gems/jekyll-4.2.0/lib/jekyll/commands/serve/servlet.rb:3:in `require': cannot load such file -- webrick (LoadError)
from /usr/local/lib/ruby/gems/3.0.0/gems/jekyll-4.2.0/lib/jekyll/commands/serve/servlet.rb:3:in `<top (required)>'
from /usr/local/lib/ruby/gems/3.0.0/gems/jekyll-4.2.0/lib/jekyll/commands/serve.rb:179:in `require_relative'
from /usr/local/lib/ruby/gems/3.0.0/gems/jekyll-4.2.0/lib/jekyll/commands/serve.rb:179:in `setup'
from /usr/local/lib/ruby/gems/3.0.0/gems/jekyll-4.2.0/lib/jekyll/commands/serve.rb:100:in `process'
from /usr/local/lib/ruby/gems/3.0.0/gems/jekyll-4.2.0/lib/jekyll/command.rb:91:in `block in process_with_graceful_fail'
from /usr/local/lib/ruby/gems/3.0.0/gems/jekyll-4.2.0/lib/jekyll/command.rb:91:in `each'
from /usr/local/lib/ruby/gems/3.0.0/gems/jekyll-4.2.0/lib/jekyll/command.rb:91:in `process_with_graceful_fail'
from /usr/local/lib/ruby/gems/3.0.0/gems/jekyll-4.2.0/lib/jekyll/commands/serve.rb:86:in `block (2 levels) in init_with_program'
from /usr/local/lib/ruby/gems/3.0.0/gems/mercenary-0.4.0/lib/mercenary/command.rb:221:in `block in execute'
from /usr/local/lib/ruby/gems/3.0.0/gems/mercenary-0.4.0/lib/mercenary/command.rb:221:in `each'
from /usr/local/lib/ruby/gems/3.0.0/gems/mercenary-0.4.0/lib/mercenary/command.rb:221:in `execute'
from /usr/local/lib/ruby/gems/3.0.0/gems/mercenary-0.4.0/lib/mercenary/program.rb:44:in `go'
from /usr/local/lib/ruby/gems/3.0.0/gems/mercenary-0.4.0/lib/mercenary.rb:21:in `program'
from /usr/local/lib/ruby/gems/3.0.0/gems/jekyll-4.2.0/exe/jekyll:15:in `<top (required)>'
from /usr/local/lib/ruby/gems/3.0.0/bin/jekyll:23:in `load'
from /usr/local/lib/ruby/gems/3.0.0/bin/jekyll:23:in `<top (required)>'
from /usr/local/Cellar/ruby/3.0.0_1/lib/ruby/3.0.0/bundler/cli/exec.rb:63:in `load'
from /usr/local/Cellar/ruby/3.0.0_1/lib/ruby/3.0.0/bundler/cli/exec.rb:63:in `kernel_load'
from /usr/local/Cellar/ruby/3.0.0_1/lib/ruby/3.0.0/bundler/cli/exec.rb:28:in `run'
from /usr/local/Cellar/ruby/3.0.0_1/lib/ruby/3.0.0/bundler/cli.rb:497:in `exec'
from /usr/local/Cellar/ruby/3.0.0_1/lib/ruby/3.0.0/bundler/vendor/thor/lib/thor/command.rb:27:in `run'
from /usr/local/Cellar/ruby/3.0.0_1/lib/ruby/3.0.0/bundler/vendor/thor/lib/thor/invocation.rb:127:in `invoke_command'
from /usr/local/Cellar/ruby/3.0.0_1/lib/ruby/3.0.0/bundler/vendor/thor/lib/thor.rb:392:in `dispatch'
from /usr/local/Cellar/ruby/3.0.0_1/lib/ruby/3.0.0/bundler/cli.rb:30:in `dispatch'
from /usr/local/Cellar/ruby/3.0.0_1/lib/ruby/3.0.0/bundler/vendor/thor/lib/thor/base.rb:485:in `start'
from /usr/local/Cellar/ruby/3.0.0_1/lib/ruby/3.0.0/bundler/cli.rb:24:in `start'
from /usr/local/Cellar/ruby/3.0.0_1/lib/ruby/gems/3.0.0/gems/bundler-2.2.3/exe/bundle:49:in `block in <top (required)>'
from /usr/local/Cellar/ruby/3.0.0_1/lib/ruby/3.0.0/bundler/friendly_errors.rb:130:in `with_friendly_errors'
from /usr/local/Cellar/ruby/3.0.0_1/lib/ruby/gems/3.0.0/gems/bundler-2.2.3/exe/bundle:37:in `<top (required)>'
from /usr/local/opt/ruby/bin/bundle:23:in `load'
from /usr/local/opt/ruby/bin/bundle:23:in `<main>'