fix(docs): Update System.env syntax for Gradle 9 compatibility

[dbwiddis]

This PR updates the documentation for plugin Gradle build files to fix compatibility with Gradle 9.

Problem

The current syntax using $System.env.VARIABLE_NAME for accessing environment variables is not compatible with Gradle 9.

This format is used for Sonatype credentials for SNAPSHOT publication. This fails with under Gradle 9.

credentials {
    username = "$System.env.SONATYPE_USERNAME"
    password = "$System.env.SONATYPE_PASSWORD"
}

See example failed workflow here.

Solution

Update to use the System.getenv() method instead:

credentials {
    username = System.getenv("SONATYPE_USERNAME")
    password = System.getenv("SONATYPE_PASSWORD")
}

Example fix which restored successful snapshots: opensearch-project/opensearch-remote-metadata-sdk#245

PRs submitted org-wide to update:

• fix: Update System.env syntax for Gradle 9 compatibility user-behavior-insights#122
• fix: Update System.env syntax for Gradle 9 compatibility sql#4090
• fix: Update System.env syntax for Gradle 9 compatibility skills#630
• fix: Update System.env syntax for Gradle 9 compatibility security-analytics#1571
• fix: Update System.env syntax for Gradle 9 compatibility security#5577
• fix: Update System.env syntax for Gradle 9 compatibility search-relevance#227
• fix: Update System.env syntax for Gradle 9 compatibility reporting#1120
• fix: Update System.env syntax for Gradle 9 compatibility query-insights#407
• fix: Update System.env syntax for Gradle 9 compatibility opensearch-system-templates#95
• fix: Update System.env syntax for Gradle 9 compatibility performance-analyzer#841
• fix: Update System.env syntax for Gradle 9 compatibility opensearch-learning-to-rank-base#219
• fix: Update System.env syntax for Gradle 9 compatibility opensearch-sdk-java#1031
• fix: Update System.env syntax for Gradle 9 compatibility opensearch-jvector#140
• fix: Update System.env syntax for Gradle 9 compatibility OpenSearch#19119
• fix: Update System.env syntax for Gradle 9 compatibility observability#1943
• fix: Update System.env syntax for Gradle 9 compatibility notifications#1069
• fix: Update System.env syntax for Gradle 9 compatibility neural-search#1521
• fix: Update System.env syntax for Gradle 9 compatibility ml-commons#4129
• fix: Update System.env syntax for Gradle 9 compatibility k-NN#2855
• fix: Update System.env syntax for Gradle 9 compatibility job-scheduler#821
• fix: Update System.env syntax for Gradle 9 compatibility index-management#1474
• fix: Update System.env syntax for Gradle 9 compatibility geospatial#791
• fix: Update System.env syntax for Gradle 9 compatibility data-prepper#6004
• fix: Update System.env syntax for Gradle 9 compatibility custom-codecs#273
• fix: Update System.env syntax for Gradle 9 compatibility cross-cluster-replication#1575
• fix: Update System.env syntax for Gradle 9 compatibility common-utils#867
• fix: Update System.env syntax for Gradle 9 compatibility asynchronous-search#763
• fix: Update System.env syntax for Gradle 9 compatibility anomaly-detection#1554
• fix: Update System.env syntax for Gradle 9 compatibility alerting#1920
• fix: Update System.env syntax for Gradle 9 compatibility flow-framework#1219