Removed default admin credentials.

[AWSHurneyt]

Description

1. Adjusted the password used by the security enabled tests based on this comment https://github.com/opensearch-project/anomaly-detection/pull/1134/files#r1462673941
2. Adjusted the developer guide based on this comment https://github.com/opensearch-project/anomaly-detection/pull/1134/files#r1462674251

Issues Resolved

#832

Check List

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[AWSHurneyt]

Need to backport to 2.x and 2.12. I do not have permissions to add labels.

@derek-ho could you help review this PR to ensure these changes meet the requirements of the change to the security plugin?

@zhongnansu could you help apply the backport labels, and review this change?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (e61b8c0) 66.93% compared to head (941fde0) 66.93%.

Additional details and impacted files

@@            Coverage Diff            @@
##               main     #837   +/-   ##
=========================================
  Coverage     66.93%   66.93%           
  Complexity      267      267           
=========================================
  Files            39       39           
  Lines          1742     1742           
  Branches        165      165           
=========================================
  Hits           1166     1166           
  Misses          453      453           
  Partials        123      123           

Flag	Coverage Δ
opensearch-notifications	66.93% <ø> (ø)

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[opensearch-trigger-bot]

The backport to 2.x failed:

The process '/usr/bin/git' failed with exit code 128

To backport manually, run these commands in your terminal:

# Navigate to the root of your repository
cd $(git rev-parse --show-toplevel)
# Fetch latest updates from GitHub
git fetch
# Create a new working tree
git worktree add ../.worktrees/notifications/backport-2.x 2.x
# Navigate to the new working tree
pushd ../.worktrees/notifications/backport-2.x
# Create a new branch
git switch --create backport/backport-837-to-2.x
# Cherry-pick the merged commit of this pull request and resolve the conflicts
git cherry-pick -x --mainline 1 7f38be12b007a8018d25a956a062cb7354a4ff46
# Push it to GitHub
git push --set-upstream origin backport/backport-837-to-2.x
# Go back to the original working tree
popd
# Delete the working tree
git worktree remove ../.worktrees/notifications/backport-2.x

Then, create a pull request where the base branch is 2.x and the compare/head branch is backport/backport-837-to-2.x.

[Hailong-am]

@AWSHurneyt backport 2.x failed, please help to do manual backport for 2.x, thanks.