[bugfix] Downgrade OTEL libraries to 1.20

[daenney]

Description

For some reason httpconv seems to have disappeared from 1.21, which results in a 1.21 runtime with 1.20 semconv/httpconv which seems to break some things.

For now, this rolls the OTEL dependencies back to 1.20 which should fix the observability issues. We'll need to take a look at how to upgrade safely and correctly in the future.

Relates to #2503.

Checklist

Please put an x inside each checkbox to indicate that you've read and followed it: [ ] -> [x]

If this is a documentation change, only the first checkbox must be filled (you can delete the others if you want).

• I/we have read the GoToSocial contribution guidelines.
• I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
• I/we have performed a self-review of added code.
• I/we have written code that is legible and maintainable by others.
• I/we have commented the added code, particularly in hard-to-understand areas.
• I/we have made any necessary changes to documentation.
• I/we have added tests that cover new code.
• I/we have run tests and they pass locally with the changes.
• I/we have run go fmt ./... and golangci-lint run.

[tsmethurst]

Nice one :)