Bump r2dbc-postgresql from 0.8.5.RELEASE to 0.8.8.RELEASE in /modules/postgresql (#4092)

Bumps [r2dbc-postgresql](https://github.com/r2dbc/r2dbc-postgresql) from 0.8.5.RELEASE to 0.8.8.RELEASE.
- [Release notes](https://github.com/r2dbc/r2dbc-postgresql/releases)
- [Changelog](https://github.com/pgjdbc/r2dbc-postgresql/blob/v0.8.8.RELEASE/CHANGELOG)
- [Commits](pgjdbc/r2dbc-postgresql@v0.8.5.RELEASE...v0.8.8.RELEASE)

Signed-off-by: dependabot[bot] <[email protected]>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

modules/postgresql/build.gradle

@@ -7,7 +7,7 @@ dependencies {
     compile project(':jdbc')
 
     compileOnly project(':r2dbc')
-    compileOnly 'io.r2dbc:r2dbc-postgresql:0.8.5.RELEASE'
+    compileOnly 'io.r2dbc:r2dbc-postgresql:0.8.8.RELEASE'
 
     testCompile project(':jdbc-test')
     testCompile project(':test-support')