[FLINK-36595][docs] Explicitly set connector compatibility as string …

[hlteoh37]

…to prevent version comparison mismatch

Purpose of the change

• Set version as string to prevent version comparison mismatch. Issue has happened for other connectors too : [FLINK-36595][docs] Explicitly set connector compatibility as string to prevent version comparison mismatch flink-connector-kafka#132
• Add supported version for flink-connector-opensearch for v1.18 and 1.19.
  • Verified artifacts available on maven: https://mvnrepository.com/artifact/org.apache.flink/flink-connector-opensearch.
  • Also release note: https://lists.apache.org/thread/v9gxnflhhsr92sf76rt58ob688nnr6mj

Significant changes

(Please check any boxes [x] if the answer is "yes". You can first publish the PR and check them afterwards, for convenience.)

• Dependencies have been added or upgraded
• Public API has been changed (Public API is any class annotated with @Public(Evolving))
• Serializers have been changed
• New feature has been introduced
  • If yes, how is this documented? (not applicable / docs / JavaDocs / not documented)

[boring-cyborg]

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[hlteoh37]

@snuyanzin it would be good to get your validation on this!

[nicusX]

Is this version correct?
In the v1.2 branch the version is 1.2-SNAPSHOT

flink-connector-opensearch/pom.xml

Line 31 in 419632c

<version>1.2-SNAPSHOT</version>