[foxy] [service] Don't use a weak_ptr to avoid leaking (backport #1668)

[mergify]

This is an automatic backport of pull request #1668 done by Mergify.
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	both modified:   rclcpp/include/rclcpp/service.hpp

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[jacobperron]

@ivanpauno Friendly ping. Looks like this needs some attention.

[ivanpauno]

CI:

• Linux
• Linux-aarch64
• macOS
• Windows

[jacobperron]

LGTM with green CI

[ivanpauno]

• Linux
• Linux-aarch64
• macOS
• Windows