fix(instrumentation-fastify): fix fastify typescript compilation issue

[pichlermarc]

Which problem is this PR solving?

• Looks like recent changes over at fastify desupported older versions of typescript, which breaks CI at the moment
• The PR pins the devDependency to 4.18.0 until Type definitions do not compile anymore when using TypeScript v4.6 fastify/fastify#4873 is resolved
  • if the resolution to this issue is that support will be restored, I'll unpin it again
  • if the resolution to this issue is that support will not be restored, I'll look into alternative ways of testing newer versions of fastify (possibly via TAV).

[codecov]

Codecov Report

Merging #1556 (e976e65) into main (8499b16) will decrease coverage by 0.79%.
The diff coverage is 88.88%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1556      +/-   ##
==========================================
- Coverage   96.13%   95.35%   -0.79%     
==========================================
  Files          14       20       +6     
  Lines         906     1184     +278     
  Branches      197      246      +49     
==========================================
+ Hits          871     1129     +258     
- Misses         35       55      +20     

Impacted Files	Coverage Δ
...strumentation-document-load/src/instrumentation.ts	98.13% <88.88%> (-0.86%)	⬇️

... and 6 files with indirect coverage changes

[pichlermarc]

@open-telemetry/javascript-approvers PTAL, this is currently blocking all PRs in contrib.