[2.x] Add README to version increment automation (#12894)

Signed-off-by: Craig Perkins <cwperx@amazon.com>

Author: cwperks

.github/workflows/version.yml

@@ -108,6 +108,7 @@ jobs:
       - name: Add bwc version to main branch
         run: |
           echo Adding bwc version $NEXT_VERSION after $CURRENT_VERSION
+          sed -i "s/$CURRENT_VERSION/$NEXT_VERSION/g" README.md
           sed -i "s/- \"$CURRENT_VERSION\"/\0\n  - \"$NEXT_VERSION\"/g" .ci/bwcVersions
           echo Adding $NEXT_VERSION_UNDERSCORE after $CURRENT_VERSION_UNDERSCORE
           sed -i "s/public static final Version $CURRENT_VERSION_UNDERSCORE = new Version(\([[:digit:]]\+\)\(.*\));/\0\n    public static final Version $NEXT_VERSION_UNDERSCORE = new Version($NEXT_VERSION_ID\2);/g" libs/core/src/main/java/org/opensearch/Version.java