[21725] Update examples CMakeList to force default CMAKE_BUILD_TYPE

[JesusPoderoso]

Description

This PR updates the examples CMakeList to build with Release CMake build type mode, if the mode has not been specified.

@Mergifyio backport 3.1.x 3.0.x

This PR is related to:

• [21725] Update example CMakeList to force default CMAKE_BUILD_TYPE Fast-DDS-Gen#416

Contributor Checklist

• Commit messages follow the project guidelines.
• The code follows the style guidelines of this project.
• N/A Tests that thoroughly check the new feature have been added/Regression tests checking the bug and its fix have been added; the added tests pass locally
• N/A Any new/modified methods have been properly documented using Doxygen.
• N/A Any new configuration API has an equivalent XML API (with the corresponding XSD extension)
• Changes are backport compatible: they do NOT break ABI nor change library core behavior.
• Changes are API compatible.
• N/A New feature has been added to the versions.md file (if applicable).
• N/A New feature has been documented/Current behavior is correctly described in the documentation.
• Applicable backports have been included in the description.

Reviewer Checklist

• The PR has a milestone assigned.
• The title and description correctly express the PR's purpose.
• Check contributor checklist is correct.
• N/A If this is a critical bug fix, backports to the critical-only supported branches have been requested.
• Check CI results: changes do not issue any warning.
• Check CI results: failing tests are unrelated with the changes.