refactor: ‎README is refactored

[strelchm]

Clarify polling strategy naming between documentation and Spring Boot config and additional readme refactoring

The polling strategy is referred to as fetch-and-lock-on-execute in the main documentation, but configured as fetch in Fluent API and Spring Boot application.properties.
Specifying fetch-and-lock-on-execute in Spring configuration results in startup failure cause the enum expects FETCH or LOCK_AND_FETCH:

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APPLICATION FAILED TO START
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Description:

Failed to bind properties under 'db-scheduler.polling-strategy' to com.github.kagkarlsson.scheduler.PollingStrategyConfig$Type:

    Property: db-scheduler.polling-strategy
    Value: "fetch-and-lock-on-execute"
    Origin: class path resource [application.properties] - 26:31
    Reason: failed to convert java.lang.String to com.github.kagkarlsson.scheduler.PollingStrategyConfig$Type (caused by java.lang.IllegalArgumentException: No enum constant com.github.kagkarlsson.scheduler.PollingStrategyConfig.Type.fetch-and-lock-on-execute)

Action:

Update your application's configuration. The following values are valid:

    FETCH
    LOCK_AND_FETCH

This inconsistency can be confusing for users. Changing the property name from fetch to fetch-and-lock-on-execute would be a breaking change for existing users - that is why the note in docs is the optimal variant.

Fixes

This PR adds explicit links between the two:

• Adds note in polling strategy section that default strategy is configured as fetch in Fluent API and Spring Boot
• Adds inline comment in Spring Boot properties example showing fetch corresponds to fetch-and-lock-on-execute
• Adds "SQL Server, MySQL v8+" to the lock-and-fetch description in the Benchmark test. In the pollUsingLockAndFetch config description, these databases are already listed.

Also additional refactoring:

• Removes duplicate polling strategy properties in Spring Boot example
• Replaces corrupted herokuapp Maven Central badge with modern Shields.io badge. Update link to point to central.sonatype.com

Documentation is updated to clarify the mapping, helping users avoid this error.

Reminders

• Update README and/or examples
• Ran mvn spotless:apply

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cc @kagkarlsson