Update fastly_exporter to 7.6.0

[github-actions]

Update fastly_exporter to 7.6.0.

What's Changed

New Features 🎉

• Extend Origin Inspector metrics with Compute@Edge and next-gen WAF support by @leklund in Extend Origin Inspector metrics with Compute@Edge and next-gen WAF support fastly/fastly-exporter#132
• New HTTPTotal Metric for all http requests by @leklund in New HTTPTotal Metric for all http requests fastly/fastly-exporter#134

Dependencies

• Bump github.com/peterbourgon/ff/v3 from 3.3.1 to 3.3.2 by @dependabot in Bump github.com/peterbourgon/ff/v3 from 3.3.1 to 3.3.2 fastly/fastly-exporter#136
• Drop vendor directory by @SuperQ in Drop vendor directory fastly/fastly-exporter#139
• Bump golang.org/x/sync from 0.2.0 to 0.3.0 by @dependabot in Bump golang.org/x/sync from 0.2.0 to 0.3.0 fastly/fastly-exporter#141
• Bump github.com/prometheus/client_golang from 1.15.1 to 1.16.0 by @dependabot in Bump github.com/prometheus/client_golang from 1.15.1 to 1.16.0 fastly/fastly-exporter#140

Full Changelog: fastly/fastly-exporter@v7.5.1...v7.6.0