Refactor: Replace hardcoded strings with constants and update dependencies

[SahibYar]

This PR introduces the following changes:

1. This PR introduces the following changes:

• Updated occurrences of HTTP methods (e.g., "GET", "POST") with their corresponding constants from the net/http package (e.g., http.MethodGet, http.MethodPost).
• Ensures consistency, improves maintainability, and reduces the risk of typos.

2. Updated dependencies:

• Ran go get -u to update the go.mod and go.sum files to their latest versions.

Why these changes?

• Using constants improves code readability and reduces the risk of errors from hardcoded values.
• Updating dependencies ensures the project uses the latest features, bug fixes, and security patches.

Testing:

• executed make all and received following final output

?       github.com/fastly/go-fastly/v9/fastly/products  [no test files]
ok      github.com/fastly/go-fastly/v9/fastly   (cached)
ok      github.com/fastly/go-fastly/v9/fastly/image_optimizer_default_settings  (cached)
ok      github.com/fastly/go-fastly/v9/fastly/products/bot_management   (cached)
ok      github.com/fastly/go-fastly/v9/fastly/products/brotli_compression       (cached)
ok      github.com/fastly/go-fastly/v9/fastly/products/ddos_protection  (cached)
ok      github.com/fastly/go-fastly/v9/fastly/products/domain_inspector (cached)
ok      github.com/fastly/go-fastly/v9/fastly/products/fanout   (cached)
ok      github.com/fastly/go-fastly/v9/fastly/products/image_optimizer  (cached)
ok      github.com/fastly/go-fastly/v9/fastly/products/log_explorer_insights    (cached)
ok      github.com/fastly/go-fastly/v9/fastly/products/ngwaf    (cached)
ok      github.com/fastly/go-fastly/v9/fastly/products/origin_inspector (cached)
?       github.com/fastly/go-fastly/v9/internal/test_utils      [no test files]
ok      github.com/fastly/go-fastly/v9/fastly/products/websockets       (cached)
ok      github.com/fastly/go-fastly/v9/internal/productcore     (cached)
==> Running go vet
==> Running staticcheck
staticcheck 2024.1.1 (0.5.1)
if command -v semgrep &> /dev/null; then semgrep ci --config auto --exclude-rule generic.secrets.security.detected-private-key.detected-private-key ; fi
                  
                  
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[kpfleming]

Thank you for the PR! While most of these changes are welcome, there are too many changes (of unrelated types) included in the PR. Please open PRs which focus on a specific type of change (replacement of constants, updating dependencies, correcting/improving comments, etc.).

Also, the go.mod is pinned to 1.20 for a reason; we do not want to require users of this module to use newer versions of Go at this time, so go mod tidy and similar commands must be run using Go 1.20 in order to preserve compatibility.