Updated labels in template files

For context: https://rust-lang.zulipchat.com/#narrow/stream/257328-clippy/topic/Label.20cleanup/near/224083870

.github/ISSUE_TEMPLATE/bug_report.md

@@ -1,7 +1,7 @@
 ---
 name: Bug Report
 about: Create a bug report for Clippy
-labels: L-bug
+labels: C-bug
 ---
 <!--
 Thank you for filing a bug report! 🐛 Please provide a short summary of the bug,

.github/ISSUE_TEMPLATE/false_negative.md

@@ -1,7 +1,7 @@
 ---
 name: Bug Report (False Negative)
 about: Create a bug report about missing warnings from a lint
-labels: L-bug, L-false-negative
+labels: C-bug, I-false-negative
 ---
 <!--
 Thank you for filing a bug report! 🐛 Please provide a short summary of the bug,

.github/ISSUE_TEMPLATE/false_positive.md

@@ -1,7 +1,7 @@
 ---
 name: Bug Report (False Positive)
 about: Create a bug report about a wrongly emitted lint warning
-labels: L-bug, L-false-positive
+labels: C-bug, I-false-positive
 ---
 <!--
 Thank you for filing a bug report! 🐛 Please provide a short summary of the bug,

.github/ISSUE_TEMPLATE/ice.md

@@ -1,7 +1,7 @@
 ---
 name: Internal Compiler Error
 about: Create a report for an internal compiler error in Clippy.
-labels: L-bug, L-crash
+labels: C-bug, I-ICE
 ---
 <!--
 Thank you for finding an Internal Compiler Error! 🧊  If possible, try to provide

.github/ISSUE_TEMPLATE/new_lint.md

@@ -1,7 +1,7 @@
 ---
 name: New lint suggestion
 about: Suggest a new Clippy lint.
-labels: L-lint
+labels: A-lint
 ---
 
 ### What it does