[receiver/prometheus] Upgrade receiver.prometheus.removeLegacyResourceAttributes feature flag (open-telemetry#39803)

<!--Ex. Fixing a bug - Describe the bug and how this fixes the issue.
Ex. Adding a feature - Explain what this achieves.-->
#### Description
This feature flag was introduced in
open-telemetry#32829
to resolve open-telemetry#32814. It was first released in `v0.103.0`, on June 19, 2024
which was 10 months ago. The feature gate is to remove deprecated
resource attributes in favor of the semantic convention equivalents.
This is a breaking change and the feature flag will now be enabled by
default.

<!-- Issue number (e.g. open-telemetry#1234) or full URL to issue, if applicable. -->
#### Link to tracking issue

open-telemetry#32814

<!--Describe what testing was performed and which tests were added.-->
#### Testing
Updated testing to account for the feature flag being enabled by
default.

Author: crobert-1

.chloggen/upgrade_prom_feature_gate.yaml

@@ -0,0 +1,32 @@
+# Use this changelog template to create an entry for release notes.
+
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: breaking
+
+# The name of the component, or a single word describing the area of concern, (e.g. filelogreceiver)
+component: receiver/prometheus
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Upgrade `receiver.prometheusreceiver.RemoveLegacyResourceAttributes` feature gate to `beta` stability (enabled by default)
+
+# Mandatory: One or more tracking issues related to the change. You can use the PR number here if no issue exists.
+issues: [32814]
+
+# (Optional) One or more lines of additional information to render under the primary note.
+# These lines will be padded with 2 spaces and then inserted directly into the document.
+# Use pipe (|) for multiline entries.
+subtext: |
+  Disable the `receiver.promtheusreceiver.RemoveLegacyResourceAttributes` feature gate to restore previous behavior. This feature gate will be removed in a future release.
+  The feature gate is used to replace the following legacy resource attributes:
+  `net.host.name` -> `server.address`
+  `net.host.port` -> `server.port`
+  `http.scheme` -> `url.scheme`
+
+# If your change doesn't affect end users or the exported elements of any package,
+# you should instead start your pull request title with [chore] or use the "Skip Changelog" label.
+# Optional: The change log or logs in which this entry should be included.
+# e.g. '[user]' or '[user, api]'
+# Include 'user' if the change is relevant to end users.
+# Include 'api' if there is a change to a library API.
+# Default: '[user]'
+change_logs: []