Changed path for path/filepath

[TiagoMHCSantana]

Required checklist

• [N/A] Sample config files updated (both /etc folder and NewDemoConfig methods) (influxdb and plutonium)
• [N/A] openapi swagger.yml updated (if modified API) - link openapi PR
• [OK] Signed CLA (if not already signed)

Description

PR seeks to solve an error thrown when using alert-template in HttpPost service running on Windows.

Context

path package cannot deal with Windows paths, this is stated right in 'Overview' section (https://pkg.go.dev/path). Instead, path/filepath package should be used.

Affected areas (if applicable):

Does not affect user-visible areas

Note for reviewers:

Check the semantic commit type:

• Feat: a feature with user-visible changes
• Fix: a bug fix that we might tell a user “upgrade to get this fix for your issue”
• Chore: version bumps, internal doc (e.g. README) changes, code comment updates, code formatting fixes… must not be user facing (except dependency version changes)
• Build: build script changes, CI config changes, build tool updates
• Refactor: non-user-visible refactoring
• Check the PR title: we should be able to put this as a one-liner in the release notes