[hotfix][build] Fix Dinky backend CI workflow with Flink 1.20

[yuxiqian]

Purpose of the pull request

This fixes Flink 1.20 packaging workflow isn't running as expected.

#3786 adds Flink 1.20 packaging profile, however it isn't running as expected, since 1.20 is interpreted as a floating value in YAML, and it decays to 1.2 at runtime.

CI workflows targeting Flink 1.20 are actually running with the following configuration:

Run ./mvnw -B clean install \
  ./mvnw -B clean install \
          -Dmaven.test.skip=false \
         -Dspotless.check.skip=true \
         -Denforcer.skip=false \
         -Dmaven.javadoc.skip=true \
         -P prod,flink-single-version,flink-1.2,maven-central \
         --no-snapshot-updates
  shell: /usr/bin/bash -e {0}
  env:
    MAVEN_OPTS: -Xmx2G -Xms2G
    JAVA_HOME: /opt/hostedtoolcache/Java_Adopt_jdk/8.0.432-6/x64
[INFO] Scanning for projects...

Brief change log

Changing flink matrix argument to '1.20' (explicitly as string) should fix this issue.

Verify this pull request

Ran backend CI tests in my own repo and it works as expected.

[aiwenmo]

LGTM

[zackyoungh]

LGTM